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How to repair paint affected by corrosion on aluminium
Paint Guide BW028
Paint job at a glance
Here's your custom paint guide
Before any project, take these things into account to get the best results:
- Factor in enough time for each stage of your project so you don't have to rush.
- Always check the weather while planning your project. Temperature, humidity and dew point affect the quality of your work.
- Check you have all the equipment, tools and products you need.
- Ensure you read all the information in the health and safety tab above.
Equipment we recommend for this guide
Equipment List 2
- Rubber gloves
- Dust mask
- Sanding paper 80 - 180 grit (various grades for surface preparation)
- Palette knife, spreader or small trowel
- Paint roller tray
- Paint rollers (suitable sizes and types)
- Paint brushes (suitable size)
- Tack rag or lint free cloth
Equipment List 3
- Safety shoes
- Face dust masks
- Hand protection (as per product SDS)
- Overalls
- Sanding machine and/or suitable sanding blocks
- Eye protection
- Specialized cleaning product
Your health and safety
Your safety is of the utmost importance to us. Ensure that you read and follow all label instructions and the safety datasheet before starting your project. Also:
Ensure the area you are working in is properly ventilated.
Ensure all products are kept out of reach of children during storage and usage.
Ensure solvent fumes and any splashing of material will not affect others nearby.
PPE (Personal Protective Equipment)
Before you begin, always refer to the label and safety datasheet for product specific information. Using the right PPE for your project is important to ensure your safety. You should always check the information from the PPE manufacturer to ensure you use the correct items for your working conditions.
Wear safety glasses at all times to protect your eyes from splashes.
Chemicals can cause skin irritation. Wear overalls and protective gloves to protect your skin and clothing.
To avoid inhalation of dust while sanding; wear a dust mask at all times.
Many products contain either solvents or other chemicals that can build up in confined areas creating a hazard and/or an unpleasant working environment. Wear suitable respiratory protection if ventilation is not sufficient or if required by the safety datasheet to do so.
See the protective equipment overview for further information.
We care deeply about the environment and we want to ensure that our products are disposed of in the correct way. With that in mind, please:
- Always refer to the product label and safety datasheet for any specific instructions.
- Ensure any waste product including cans, rags, brushes, roller sleeves masking tape and paper, gloves and overalls are disposed of according to local regulations.
- Do not allow any waste material such as unused paint to enter any waterways.
- Ensure waste and empty containers are disposed of in accordance with local legislation guidelines and national regulations.
For more information or any special waste regulations, please contact your local waste regulation authority.
Step 1 Cleaning painted aluminium
1.1 Mask off
- Mask the repair area to protect the surrounding surface.
1.2 Rinse
- Rinse the surface with fresh water to remove loose dirt, growth or any other contamination. You can also scrub it with a hard sponge.
Show tips from pros
- To tell if the surface is properly degreased, the water should spread across the surface while flushing. Small droplets of water are an indicator that the surface isn’t fully degreased. If so, repeat the cleaning process.
- Never clean antifouling with solvents as this can damage the surface.
- High pressure washing removes most of the growth in an effective way.
- Pay attention on the distance between the surface and the high pressure washer. Some machines have enough power to remove the paint system.
- Special attention should be paid to clean around the waterline or other areas with visible contamination using an abrasive pad with water.
- Masking the surrounding area helps to prevent contamination spreading to other surfaces.
Show equipment you'll need
- High pressure washer
- Extension for cleaning tool
- Sponge and/or cloths
- Rubber gloves
- Safety shoes
- Overalls
- Eye protection
- Specialized cleaning product
Step 2 Sanding paint in bad condition on aluminium
2.1 Remove flaking paint
- Scrape away any loose paint that’s in bad condition. This might mean you’ll have to remove all the paint down to the bare substrate. It’s ok to save any layers that are in good condition.
2.2 Remove corrosion
- Any corrosion will require careful handling to remove it (e.g. rust or metal oxides) down to good sound metal. For small areas, grinding or sanding is suitable. For larger areas, blasting is the best method but should only be carried out by a professional.
2.3 Complete repairs
- At the centre of the repair:
- If sanding to existing primer, use 60-120 grit paper.
- If sanding to bare substrate, use 24-60 grit on the bare metal.
- If removing any antifouling layers, this must be done by wet sanding.
- If sanding to existing primer, use 60-120 grit paper.
- From the edge of the repair:
- Continue sanding using 60-120 grit paper to transition to the existing primer system.
- Continue sanding using 60-120 grit paper to transition to the existing primer system.
- This will minimize the risk from the touch-up primer being applied over the existing antifouling, which can cause problems if a 2-pack primer is used.
- Single pack paint must only be repaired or overcoated with other single pack paints.
2.4 Prepare remaining surface
- Sand with 120 grit. Please be aware that antifouling must only be wet sanded to minimise exposure to biocides.
2.5 Final sanding
- Any bare aluminium that’s left needs to be sanded to help the paint scheme adhere to it. Sand well using 24-60 grit (aluminium compatible) to achieve a rough surface profile to improve adhesion.
2.6 Clean surface
- Remove any sanding residues with clean compressed air, sweep the surface with a clean brush, or vacuum clean for the best results.
2.7 Application
- Aluminium starts oxidizing as soon as it is exposed, so the first coat of primer on bare aluminium should be applied within 6-8 hours after you’ve finished the blasting or sanding.
Show tips from pros
- For bare aluminium, grit blasting is the best preparation method as it creates an ideal profile for paint adhesion. However, this should only ever be done by a professional. Make sure they only use aluminium compatible grit and not copper slag abrasives, as these will promote serious corrosion.
- The paint can be removed using a suitable paint stripper (aluminium compatible), or by sanding with 80-120 grit. Please be aware that antifouling must only be wet sanded to minimise exposure to biocides.
- Pay extra attention to the paint film in areas with rivets and weldings.
- Inadequate preparation can cause future paint system failures.
- If you use a paint stripper, it must be aluminium compatible.
- For optimum results, use a sanding machine.
Show equipment you'll need
- Sanding paper 24 - 120 grit (various grades for surface preparation)
- Vacuum cleaner (or compressed air)
- Cleaning thinner
- Rubber gloves
- Dust mask
- Overalls
- Sanding machine and/or suitable sanding blocks
- Eye protection
Step 3 Applying fillers and epoxies
If you’re doing repair work, carefully inspect for any damage including fine cracks and any material that’s poorly adhered. Small repairs and filling on new boats can be easily tackled. However, any work that’s over a larger area or might affect the structure or hull integrity, should be referred to a professional for proper assessment.
It's recommended to apply a layer of primer before application of filler. Please see step relating to the application of primer.
Important note relating to filling and fairing: If you are intending to carry out any filling and/or fairing work please read the following note very carefully.
On all bare substrates a coat of primer MUST be applied before proceeding with any filling and/or fairing. Once this work has been completed then the second and subsequent coats of primer should be applied before proceeding with application of the rest of the chosen above or below waterline system.
3.1 Mask off
- Mask off any areas that are not going to be painted such as topsides on the waterline with suitable masking tape.
3.2 Mixing
- Most fillers used in the marine environment are two pack epoxy based fillers. For more specific information, please refer to the datasheet or the label on the can.
- Two pack products:
- Measure out part A and part B onto a clean mixing board using the specified mix ratio.
- Using a palette knife or spatula, mix the two components together until you achieve a uniform colour with no streaks.
- Measure out part A and part B onto a clean mixing board using the specified mix ratio.
- To maximise the working time of the product, spread it out over the mixing board to prevent it curing too quickly.
3.3 Application
- The best way is to apply filler using a palette knife, spatula or any other suitable tools in smooth, thin layers. Try not to apply too much in one go. You may need to apply multiple layers.
- Allow it to cure following the instructions on the datasheets or label. Two pack products have a limited pot life when mixed. Refer to label or technical data sheet for further information.
3.4 Sanding and overcoating
- After the filler has been applied and has cured, you need to smooth the surface using a sanding block that will ideally be larger than the filled area. The larger the filled area, the larger the block will need to be to sand it.
- Initial sanding can be completed with 80 grit. If additional filler needs to be applied, remove the dust from the surface and follow the previous step. Once the surface is smooth and no more filler is needed, final sanding can be completed with 120-180 grit.
Show tips from pros
- Epoxies must be mixed in the proper ratio. Add too much curing agent and they will leave a sticky film on the surface that’s not suitable for overcoating. Too little curing agent will weaken the filler and cause it to crumble later on.
- When measuring out epoxy filler that has to be mixed 2:1 by volume, the easiest way is to measure out three equal volume piles (2 of the base and 1 of the curing agent/hardener) rather than trying to gauge if one is twice the other.
- Metal measuring spoons of various sizes that you can buy from the supermarket, are ideal for measuring small quantities of product.
- Below the waterline, epoxy fillers must be used. Polyester or car fillers should not be used as they have a greater tendency to absorb water.
- Never add thinners to fillers as this will seriously affect the integrity of the cured product.
- Old plastic credit cards make excellent application and smoothing tools for smaller areas of filler.
- When sanding fillers, it’s very easy to inadvertently sand surrounding areas forming a lower area that will show right through to the finish. Be careful to avoid this.
Show equipment you'll need
- Sanding paper 80 - 180 grit (various grades for surface preparation)
- Palette knife, spreader or small trowel
- Safety shoes
- Face dust masks
- Hand protection (as per product SDS)
- Overalls
- Sanding machine and/or suitable sanding blocks
- Eye protection
Step 4 Applying underwater primers
Priming is a vital part of the painting process as it’s the base on which the rest of the system will be applied. If the base isn’t good and sound, then the overall system won’t be either.
Important note relating to filling and fairing: If you are intending to carry out any filling and/or fairing work please read the following note very carefully.
On all bare substrates a coat of primer MUST be applied before proceeding with any filling and/or fairing. Once this work has been completed then the second and subsequent coats of primer should be applied before proceeding with application of the rest of the chosen above or below waterline system.
4.1 Mask off
- Mask off any areas that are not going to be painted such as topsides on the waterline with suitable masking tape.
4.2 For small repairs
- Use a suitable sized brush or roller to suit the size of the repair area.
- Small areas less than about credit card sized can be primed using scrap pieces of suitable foam or a foam brush and dabbing the primer on and using a new piece for each subsequent coat.
- With any repair, the area to be repainted will need to be larger than the original damage so the paint system can be transitioned gradually to ensure a smooth surface.
- As a general rule, each layer of paint should overlap the previous coating by 5%-10%. Start by painting the affected area and move out, following the application steps below.
4.3 Mixing
- Single pack products:
- With single pack primer mix the paint thoroughly with a stirring stick ensuring that any settlement is well mixed in.
- With single pack primer mix the paint thoroughly with a stirring stick ensuring that any settlement is well mixed in.
- Two pack products:
- If you are using a 2 pack primer, mix the individual components thoroughly. Two pack products have a limited pot life when mixed. Refer to label or technical data sheet for further information.
- Combine the two products and mix the paint thoroughly as indicated on the datasheet or the label on the can.
- If you are using a 2 pack primer, mix the individual components thoroughly. Two pack products have a limited pot life when mixed. Refer to label or technical data sheet for further information.
- In some instances, it's necessary in to add solvents such as to aid application or improve penetration of the first coat on wood. For the recommended solvent and the amount you need to add, please refer to the datasheet or the label on the can.
4.4 Application
- Pour the paint into a clean roller tray or paint pot.
- Start somewhere where any application overlap will be less noticeable such as on the bow or the corner of the transom.
- For large surfaces, it's advised to use a roller as it's quicker and will achieve a uniform finish.
- If you’re applying product with a brush, a good technique is the criss-cross method. Paint is applied to the surface with diagonal brushing from left to right. It’s then spread further with horizontal strokes before finally laying off with light vertical strokes. This applies a uniform layer of paint and allows the brush marks to flow out better.
- Apply sufficient paint for the product to flow, but not so much to make it sag or run as it can be hard to remove them later. If you do see any sags or runs, brush them out as soon as possible.
4.5 Overcoating
- Most products will require more than one application or coat as per datasheet or label on the can.
- If any filling is required this should be done after the first coat of primer is applied.
- If additional coats are required, follow the recommended overcoating times as per the datasheet or label on the can.
- Most primers have extended recoat times with themselves, so you can apply the required number of coats without having to sand in-between. If you exceed the maximum overcoating time, you will need to sand the surface with 120-180 before applying the next coat. When applying antifouling over primers some have limited recoat times so check labels and technical data sheets for any primer used to ensure you are within the recoat window for the antifouling product being applied.
- Product performance is based upon the film thickness, so please make sure the recommended number of coats are applied.
Show tips from pros
Working with a roller:
- Applying paint with a roller is a fast method of covering large areas.
- For most applications, a 5-6 mm nap felt or mohair roller is suitable. Before using them, wrap masking tape around a new roller and then pull off to remove any loose fibres.
- If you're trying to achieve a smoother finish, you could use a high density closed cell foam roller. This may lead to a thinner coat of product, so you may need to apply an extra coat.
- Some rollers may be affected by solvents in the product and can swell during use. When they become too soft to use, or look like they are breaking up, replace them with a new one.
- When using a roller and tray, it’s a good idea to keep the tray covered loosely to avoid the wind, sun or air creating a skin over the paint during use.
- If the area to be painted is very small you can obtain smaller rollers from various hardware stores. Some are often called radiator rollers that are very good for small and difficult to get to areas.
Working with a brush:
- Brushes should be medium to large width typically 75-150mm with long flexible bristles.
- A smaller brush will be used for painting difficult to reach areas.
- Wash your brushes with the appropriate solvent and dry them thoroughly before using to avoid contamination.
- The quality of brushes required for priming is less critical than those used for applying undercoats or finish coats.
- To minimise brush marks hold the brush at a 45 degree angle to the surface.
- To clean brushes, place some thinner inside a suitable container so you can clean it if the bristles start to clog due to curing or thickening of the paint.
Other useful tips:
- If you’re getting runs as the paint is applied, then it’s either too thin, or you’re applying too much.
- Avoid using paint directly from the can as this might introduce contamination and prematurely age the paint from solvent evaporation. Instead, pour what you’d expect to use in 30 minutes into a separate container.
- Old jam jars or clean dry tin cans are useful for mixing paint. Also, metal measuring spoons of various sizes you can buy from any supermarket, are ideal for measuring small quantities of paint and hardener for the smaller jobs.
- For primers that you’re applying with antifouling, you need to ensure that the interval time between the end of the application of the epoxy primer and the first coat of antifouling is no longer than stated on the datasheet or label. This is especially true with epoxy based primers. If you miss this interval, you’ll have to either sand the primer or apply another coat and ensure you don’t miss the overcoat interval the second time around.
- If any of the applied coats develops runs or sags (or has contamination in it) that you need to sand out, use 120-220 grit paper. Start with 220 grade and if it keeps clogging change to 120. Any coarser and you run the risk of removing too much product and/or sanding through to the substrate. Refer to label data for self-on-self recoating information, plus antifoulings over coating times.
Show equipment you'll need
- Sanding paper 120 - 220 grit (various grades for surface preparation)
- Paint roller tray
- Paint rollers (suitable sizes and types)
- Paint brushes (suitable size)
- Tack rag or lint free cloth
- Safety shoes
- Face dust masks
- Hand protection (as per product SDS)
- Overalls
- Sanding machine and/or suitable sanding blocks
- Eye protection
Step 5 Applying antifouling paint
5.1 Mask off
- Mask off any areas that are not going to be painted such as topsides on the waterline with suitable masking tape.
5.2 Mixing
- Single pack products:
- With single pack antifoulings mix the paint thoroughly with a stirring stick ensuring that any settlement is well mixed in. Stir frequently during application as the heavy compounds may settle during use.
- With single pack antifoulings mix the paint thoroughly with a stirring stick ensuring that any settlement is well mixed in. Stir frequently during application as the heavy compounds may settle during use.
- Two pack products:
- If you are using a 2 pack antifouling, mix the individual components thoroughly (if both are liquid). For some products the second component can be a powder.
- Combine the two components and mix the paint thoroughly as indicated on the datasheet or the can.
- If you are using a 2 pack antifouling, mix the individual components thoroughly (if both are liquid). For some products the second component can be a powder.
5.3 Application
- Antifouling paints are formulated to be applied as they are, so there’s no need to add thinners to them. However, there are some occasions where a small amount of solvent can be added to aid application. Diluting the antifouling too much will affect the amount of product that's applied to the surface and in turn its long term performance. Please see tips for more details.
- Pour the paint into a clean roller tray.
- Roll or brush the paint onto the surface ensuring that a good even film of paint is applied. For additional information related to your specific product, please refer to the datasheet, the label on the can or further information on this website.
- Allow the paint to dry as per datasheet or label on the can.
5.4 For small repairs
- Before applying the first coat of antifouling to the general area, apply an initial coat over the repair area and out a further 5-10% to ensure the primer edges are well coated before you continue with the full application.
- The newly applied antifouling will display a different colour to the existing antifouling even when they are the same brand, type and colour. After immersion it should revert to much the same colour.
5.5 Overcoating
- Most products will require more than one application or coat as per datasheet or label on the can.
- If additional coats are required follow the recommended overcoating times as per the datasheet or label on the can.
- It is good practise to apply an extra coat to leading and trailing edges such as the waterline, trim tabs, outdrives, keels and rudders. These areas experience more water flow and turbulence and hence more wear on the paint surface.
5.6 Finishing up
- Be sure to follow immersion times carefully. If you don’t, it could result in the detachment, blistering or cracking of the antifouling. The marine environment is harsh so the paint must be allowed to dry thoroughly before immersion.
- Once the paint is dry, remove all masking tape.
- To clean equipment, use the recommended solvent from the datasheet or the label on the can.
- For waste disposal please refer to the top of this guide.
Show tips from pros
Working with a roller:
- Applying paint with a roller is a fast method of covering large areas.
- For most antifoulings applications a 7-9 mm nap solvent resistant mohair roller is suitable. For thinner antifoulings, a 5-6 mm nap solvent resistant mohair roller, or a high density closed cell foam roller should be used.
- If rollering with felt or mohair rollers, wrap masking tape around a new roller and then pull off to remove any loose fibres.
- Some rollers may be affected by solvents in the product and can swell during use. When they become too soft to use, or look like they are breaking up, replace them with a new one.
- When using a roller and tray it is a good idea to keep the tray covered loosely to avoid the wind, sun or air creating a skin over the paint during use.
Working with a brush:
- Brushes are best used for small areas. As the finish achieved is unlikely to be as good as a topside finish, the type of brush you use isn’t so critical.
- Wash your brushes with the thinner and dry them thoroughly before use to avoid contamination.
Other useful tips:
- If it is particularly hot or cold weather conditions, you can add a small amount of suitable thinner (no more than 10%) to help ease the application. Check labels and data sheets to ensure that the correct thinners are used as some antifouling products are water based.
- Never add anything else (additives, etc.) to an antifouling as this won’t improve performance of your coating. It may adversely affect the chemistry of the paint formulation. In many regions or countries it is also illegal to do this.
- Do not mix different antifoulings because they could be based on a different chemistry and may impact performance.
- Don't leave masking tape on longer than needed as it will be difficult to remove.
- Ensure you apply the correct amount of paint as stated in the datasheet.
Show equipment you'll need
- Solvent mask
- Paint rollers (suitable sizes and types)
- Paint brushes (suitable size)
- Safety shoes
- Hand protection (as per product SDS)
- Overalls
- Eye protection
We're here to help
We're ready to help. Our friendly help desk is open weekdays 8:00 - 16:30
Call 01489 775062
Send an e-mail to iyp.uk@akzonobel.com
Whether you call or e-mail, please mention your paint guide reference: BW028
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