How much gelcoat should I remove before redoing the entire hull?
I have a 1972ish sunfish I am planning to re-gelcoat the entire bottom of the hull. I made one fiber glass repair where there was a crack through the hull, I took that area down to bare fiberglass before repairing. In another area cracks in the gecoat turned out to be related to a thick layer (3/4 of an inch, 2 cm) of bondo over an old repair. I sanded that area to the fiberglass, unsure if I am going to add more fiberglass, but not replacing the bondo that thickly.
Where there are scratches or spider crackes I sanded until they were gone, mostly there is still a layer of gelcoat, in few place there is a little fiberglass showing. For the most part 95% of the hull has a layer of gelcoat sanded with 60 grit.
I have purchased two quarts of white exposy gelcoat, one laminating and one waxed for the final coat.
Some research I have done suggest the thick gelcoat is part of the reason for spider cracks (when the fiberglass flexes the gelcoat cracks).
My plan at the moment is, a coat of laminating gelcoat on bare fiberglass, a second coat of laminating on the entire hull, a third coat of waxed/final gelcoat on the entire hull, sand with 400-600 grit and put the trim back on. All gelcoat will be applied with a brush.
Should I sand the entire hull until the gelcoat is mostly gone? If I consider existing gelcoat as one layer, what the is the maximum number of coats that should be applied over any area of the hull?
1 answer
Talking of brushing coats make things a bit difficult because you can apply thick or thin depending on your "hand" and if sanding is required, which could mean even 4 coats, you have some coarse sandpaper scratches to fill. Quality is also a factor in amount of solids, some cover little. You are trying to do what is normally done for a patch but for the whole hull so it might end with cracks over the years.
Since you opted for a final layer of waxed (you could have used all non waxed and sealed with PVA to allow curing) you will have to wait for full curing, and then cleaning the wax if you ever find you need to cover other scratches.
You should see how thick is the gelcoat already on from the areas you sanded through it, but other than that you can't treat the old gelcoat as X thickness, you don't know how it was applied.
If in good conditions you could always patch the bare spots with new gelcoat (non waxed) cover with PVA for proper curing, wash the PVA off and sand to feather-in the patches and go over the whole hull with a fine grit to eliminate scratches and prep it for a final coat of waxed (waxed over waxed to be sanded means a lot of time for curing and cleaning in between coats). Perfect surface preparation from the beginning and maybe even spraying would mean no need for sanding so no extra thickness to be applied and just allow for final polish.
I probably would do 2 coats if I were to use a roller on perfectly prepped fiberglass surface (I don't brush gelcoat, no sanding on it), and I would be on the thick side, think that usually you are around 0.5mm for gelcoats, you are just getting UV protection. I would also eliminate all the gelcoat that there is, if nothing even just to be sure of what is underneath, it's quite a bit of work but if there was something hiding you are going to find it. Eliminate all the bondo (it's ok on a car, not on a boat... that thick not even on a car really...) and use proper fairing compound (or resin+microspheres) instead.
Don't worry if it was polyester gelcoat over epoxy resin (formulated for that application), just for the gelcoat on an hull that is not constantly in the water adhesion will be ok (there was some testing done on that), but any repair will need to be fully cured and properly prepared (unless you are hotcoating over polyester resin).
For a whole hull though gelcoat is a lot of work and you would get the same service from paint in a much easier way, it's not a repair where you are trying to match the existing finish after all. Paint requires its own maintenance, you will have to repaint at one point, and future repairs will need to take in account that there's paint on the hull but it's less work with good results.
And be aware that bad quality gelcoats will start going to chalk rather early and those cannot be re-buffed.
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