Expert advice

How do I apply a two part polyurethane over an existing unknown coating?

How do I apply a two part polyurethane over an existing unknown coating.

Two component polyurethane finishes will offer excellent abrasion resistance along with outstanding color and gloss retention. However, the solvents used are quite aggressive - so aggressive that they can actually dissolve existing finishes.

Before jumping right into the job, it would be best to test the existing finish to ensure that there are no compatibility related challenges. The best way to test the surface would be:

  • Scuff an inconspicuous section of the existing paint with 120-grit paper.
  • Purchase some 2316N from a local marine supply store.
  • Saturate a rag in the 2316N and place it onto the sanded surface.
  • Place a piece of plastic (possibly a Ziploc sandwich bag) over the saturated rag.
  • Tape off all edges of the plastic to ensure that no solvents begin to evaporate.
  • Allow the rag to remain present for a full 24 hours.
  • Once the rag has been removed, inspect the surface for any signs of bubbling/blistering/wrinkling/detachment:
    • If any are showing it would be in your best interest to completely remove all coatings before painting
    • If the surface is in the same shape which it was prior to the test (well adhered and intact) then you would likely be ok to apply primer and finish

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