Creating a beautiful, gleaming finish to your boat is a process that requires a lot of patience, care and attention. But the time spent will definitely be worth the effort.
There are lots of varnishes to choose from. To help you decide which one is right for your job, the first step is learning the differences between them. Essentially there are three main categories of varnishes that will cover all marine uses: conventional, two-pack polyurethane and oil-modified.
As to where to use which types, it’s really a case of what area you intend to varnish. For most areas, single-pack varnishes will work very well but wherever there is high abrasion, or if chemicals are involved such as around galley areas, then a two-pack polyurethane type material would be best. For smaller, hard working dinghy type boats it’s probably best to use a two-pack polyurethane type varnish to give extra resistance to knocks and bangs.
Conventional marine varnishes
Based on a variety of alkyd resins, similar to those used in enamel paints, these products are manufactured from vegetable oil, which is critical in creating exterior durability and a nice clear finish. Modifying them with polyurethane speeds up the drying rate and provides a slightly tougher product once cured.
Tip: Use a reduced gloss varnish if you’re after a smooth satin finish to make timber look more natural.
Two-pack polyurethane varnishes
These varnishes use a chemical curing mechanism that creates a very high film toughness once cured, and good chemical resistance. They’re not as easy to use, but they do give exceptional performance and will last far longer than a single-pack product. Tip: They perform even better with UV absorbers added.
These products use a variety of natural oils to enhance water resistance, timber penetration and flexibility improving resistance to cracking and crazing.
In the marine area, the oil of choice is tung oil also known as China wood oil.
Tip: Make sure you check the amount of oil in a product, as some only have small amounts and can still claim to contain tung oil, but not enough for best results.
The importance of UV absorbers
For marine use, all of the above varnishes benefit from the addition of UV absorbers or antioxidants. These absorb incoming light, which would otherwise break down the resin polymer network and lead to rapid cracking and flaking.
Tip: As it’s virtually impossible to prevent these additives from being slowly leached out of the surface by the effects of the weather, it’s vital to apply fresh coats on a regular basis.
Things to watch for
Water behind varnish films can show up as a white bloom that looks like it’s on the surface but can’t be removed. There’s also a chance that either black fungal/algal or metallic type black spots and staining will develop under the varnish film. This is especially the case where the environment is mostly fresh water, ie. vessels on rivers or lakes.
Problems with oily timbers
Some timbers, such as teak and iroko, contain high levels of natural oils which make them ideal for marine use. However, before varnishing, this oil must be removed from the surface otherwise the varnish may well detach itself at some later stage.
Tip: As with any timber, never leave it bare for any longer than necessary as it will absorb moisture from the atmosphere, especially if it’s not under good cover.
Step 1: Preparation
The first step is to prepare the timber really well. If it’s a new job, make sure you find good seasoned timber. If it’s a refurbishment, ensure all the old varnish is sanded back to good sound timber or varnish base. When doing this, use scrapers with their cutting edges rounded at the ends to prevent accidentally gouging the timber. The best sandpaper to use is aluminium oxide or dri-lube stearated paper ranging from 80-320 grit.
Any black mould spots need to be treated with a mould remover. Bleached and darkened timber will need to be restored with an Oxalic Acid type timber product, or any one of the numerous ones available on the market. The aim is to obtain a uniform timber appearance free from mould and bleached patches etc.
Once done, the timber requires another thorough sand down. Remember to only ever sand with the grain. If you sand across the grain, it will show up as darker lines or gouges when you apply the varnish, so it’s worth the effort. Sand progressively with finer papers aiming to eliminate sanding marks with each finer paper used.
Machine sanding can be used on larger areas, but make sure you don’t cut into the grain too far by using excessive pressure. The deeper you cut, the more you’ll need to resort to hand sanding later on.
Once you’ve sanded the timber and created a superb smooth finish, dust it down as many times as necessary to ensure it’s totally dust free. The best way to feel dust is to wipe a clean, dry, grease free hand very lightly across the surface.
Step 2: Getting ready
Before varnishing remember to change into dust-free, lint-free clothes. Don’t wear things like a woollen jumper, as the hairs and dust will easily contaminate the varnish.
Good brushes are essential. Preferably well-used brushes that have good tapers to their bristles and are soft and ultra clean. New brushes tend to drop hairs and should be used for priming and/or undercoating until well run in.
Tip: To get your brush in the best condition for painting, wash it in warm soapy water and then rinse in fresh water the night before and hang in an airing cupboard to dry. Ensure the bristles are carefully held together so they don’t splay out whilst drying. Lastly, make sure you have various sizes for cutting in around windows and other such features.
Step 3: Prepare the varnish
Firstly, don’t shake the can. Unlike paint there’s nothing to settle out and shaking only makes the varnish foam and will make it difficult to apply.
The only exception to this rule is for a satin or semi-gloss finish where some slight settling of the gloss reducing materials may have occurred. In this case use a wide spatula or stirring stick and with a gentle lifting action stir any settlement back in.
Work out how much varnish you need for each varnishing session and carefully decant it into a clean plastic, paper, glass or tin container. Re-lid your varnish and put it away to avoid dust and rubbish getting into the can.
The best time to varnish depends on the area to be coated. If you apply varnish as the heat of the day is building, ie. from after lunch to mid-afternoon, air trapped in the timber can expand and create small air bubbles in the varnish surface. By applying the varnish as the temperature is dropping, the cooling of the timber will draw the varnish into the timber, giving a better seal. Tip: One vital rule to remember though is: try not to varnish in direct sunlight.
Step 4: Apply the varnish
The first coat of varnish should be thinned, normally around 15 % but see label for exact details. Thinning will ensure that the product can flow, wet out and penetrate the timber more easily helping to seal the surface.
After this priming coat, it’s a good idea to lightly sand any of the raised grain. Just be careful not to break through to the timber underneath. 220-320 paper should be fine for this purpose. Dust down and then start applying varnish unthinned.
As a rule, when applying varnish or paint, apply the product to the surface and work the brush back into the wet paint already applied. Work as quickly as possible to ensure you keep a wet edge. As you deposit the varnish, solvent will evaporate and the varnish you just applied will start to get slightly sticky. For a few minutes, you can lap new work into the old, but don’t be tempted to go back over an area once it has started to set. If you do this, you will likely end up with a messy, brush marked area that’s almost impossible to sand away.
If you have a piece of timber to varnish with a flat area and an edge detail, varnish the edge first for a distance then go back and varnish the flat portion for the same distance. Go back over the edges lightly to remove any runs or build up of varnish at the edge that may have developed. Then work your way along the rest. On vertical work, try and brush in a vertical direction wherever possible. This will reduce the chance of runs and sags.
Pro tip: If you miss an area, leave the varnish to dry and sand this particular area so it’s smooth around the edges. Then varnish the whole surface as normal.
Pro tip: Make sure you apply thin layers of varnish. Thin coats will cure better and ensure better sanding.
Step 5: Sand between coats
After a couple of coats, you may want to sand the surface lightly to get rid of the inevitable bits that seem to fall into the surface. 220-320 paper should be used for this. This should take sufficient varnish off the high spots, leaving the low spots which will help fill the grain of the timber. The aim is to sand back to a good finish each time and not rely on heavy application of product to bury any defects.
Try not to remove excess varnish along corners and edges where it’s easiest to sand. This will create thinner areas which will most likely fail quicker.
Wet sanding will give a smoother finish than dry sanding but is obviously slightly messier. However, it does have the advantage of not creating lots of dust that will float around and settle back into your next coats. Wet sanding also has a couple of other advantages. It is less aggressive than dry sanding as the water lubricates the surface. This does help prevent cutting through on edges and exposing raw timber. Secondly, it allows the sanding of a varnish film that perhaps is not as hard as it should be because it is being sanded slightly too early.
Dri-lube papers sand better than standard paper and don’t clog so readily. But when fine sanding, they still tend to score the surface unless care is taken to use fresh paper regularly.
Important: After sanding, wash down and allow to dry thoroughly.
Step 6: Final coats
Repeat the above steps until you reach the gloss and piano finish you’re after. To sand off any coats, use 320-360 paper or finer as needed. Remember that sanding between coats will remove product so apply extra coats to allow for this.
When applying varnish on exterior surfaces, remember that you don't start counting the number of coats applied until the grain has been totally filled with varnish. The reason is that you need to fill the low areas, so that the high spots end up with the right amount of varnish. If you don’t, the varnish will fade and crack along the grain where the film wasn’t thick enough.
Tip: If you are striving to achieve a piano-like finish, chances are you will be applying more than the recommended minimum number of coats. With varnish work, more is definitely better.
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