Should we sand through gelcoat to fiberglass

[wbmcalli130]

Hello all. First post from a new owners of a 1980 1700 Boler. This is a full renovation. Top to bottom. Interior is fully gutted and exterior will need a ton of fiberglass work. We jumped in head first so I suspect we will be getting really good use out of the forum.

My wife and I are having our first debate regarding the renovation. It is related to the exterior of the fiberglass shell. There is at least 2 coats of junk paint on top of the gelcoat. The plan is to sand down off the excess paint before painting with a high quality paint. Here is the question:

Should we sand beyond the paint and through the gelcoat to the fiberglass?

I believe we should because it will give us a more consistent surface that we can appropriately prime when we repaint. Also, there are a lot of holes (such as those from rivets) and some cracking that will need to be repaired. Additionally there several large cut outs areas that I will want cover with fiberglass paneling so I will need to get down the the fiberglass to appropriately patch those areas anyway.

My wife, on the other hand, thinks this will just take more time unnecessarily and potentially do damage.

Our ultimate goal is to turn this thing into a head turner so I’m willing to put in the exterior work if it helps accomplish that goal.

Thoughts from the crowd?

[John in Michigan]

General practice is to remove paint, but not gelcoat that is in good condition. Cracked, chipped, holed gelcoat needs to be ground out, beveled out and repaired.

I have found an excellent, experienced practitioner on YouTube who goes by the name BoatworksToday. You can search for his channel. He has excellent detailed videos where he explains how to repair cracks, chips, and holes, as well as how to repair larger areas.

[Raspy]

Gelcoat that is in good condition, de-waxed and scuffed, will accept paint just fine and will have a smoothness that you will work hard to match if you sand it off. Avoid the extra work of sanding good gelcoat off and then trying to match that smoothness. Each of the other situations, such as hairline cracks, impact damage and hole filling will have their own techniques for matching the surrounding area and retaining structural strength. You might use polyester resin, or epoxy in a penetrating viscosity, a putty or mixed with a thickener/filler. My preference is epoxy because of the many different types, viscosities and cure times available. Epoxy works very well over polyester, but once you start with it, you should stay with it, because polyester does not work well over epoxy.
Sounds like a fun project for the two of you to work on together and enjoy later.

[Mike_L]

I've had some experience with fiberglass boats. If the gel coat is solid and undamaged I would sand it lightly for adhesion. If it is cracked or pitted you may need to sand it more to get rid of them. Cracks and pits likely contain dirt/dust in them which needs to be removed before refinishing. Then wash it well. If the surface isn't smooth a high solids primer will help. If you want the paint job to last and look good, proper prep is essential.

[Jim Bennett]

As everyone has mentioned it is fine to paint on the gelcoat given their guidelins. The reason being is that gelcoat is a resin and not a paint, pretty much like the resin in the fibreglass itself.

[Raspy]

And once you sand through the gelcoat resin, you are into chopper-gun fiberglass and resin that was sprayed against the inside of a gelcoat coating. The gelcoat was the first step in building up the thickness of the hull inside the mold. This all means the fiberglass is not so smooth or easy to make smooth enough to match the outer gelcoat finish that we look at when looking at a nice fiberglass trailer's finish. A lot of work goes into the inside of the mold, to make the outside of the gelcoat as smooth and beautiful as possible.

[ststefan]

Originally Posted by Raspy

And once you sand through the gelcoat resin, you are into chopper-gun fiberglass and resin that was sprayed against the inside of a gelcoat coating. The gelcoat was the first step in building up the thickness of the hull inside the mold. This all means the fiberglass is not so smooth or easy to make smooth enough to match the outer gelcoat finish that we look at when looking at a nice fiberglass trailer's finish. A lot of work goes into the inside of the mold, to make the outside of the gelcoat as smooth and beautiful as possible.

Since you have gutted the entire trailer, I would fill all the rivet holes and fiberglass blocks to the interior as attachment points for all the cabinets, seating and beds. It makes for a much cleaner looking exterior.

[stevebaz]

If you remove all the gellcoat you get down to resin and glass fibers. Resin sands easily but glass sands at a different rate. If your not carefull your primer coat will print through the glass fibers. Then you have to put sucessive coats of surfacer to get a smooth consistant surface suitable for paint. Patches need to have gelcoat removed for proper repair but only remove what you need to do the repair. Then start spraying surfacer on top. And blend into the rest of the gellcoat. Since your painting instead of gellcoat use a epoxy sealer before you paint for a consistant surface on the whole trailer. That way you dont have any edge lift or material compatability issues where you cover old to new. I would not sand off the old paint. Use a citrus stripper. The walls of the trailer are very thin and the gellcoat is very,very thin.

[SteveV]

Originally Posted by stevebaz

If you remove all the gellcoat you get down to resin and glass fibers. Resin sands easily but glass sands at a different rate. If your not carefull your primer coat will print through the glass fibers. Then you have to put sucessive coats of surfacer to get a smooth consistant surface suitable for paint. Patches need to have gelcoat removed for proper repair but only remove what you need to do the repair. Then start spraying surfacer on top. And blend into the rest of the gellcoat. Since your painting instead of gellcoat use a epoxy sealer before you paint for a consistant surface on the whole trailer. That way you dont have any edge lift or material compatability issues where you cover old to new. I would not sand off the old paint. Use a citrus stripper. The walls of the trailer are very thin and the gellcoat is very,very thin.

^^^^^^^
Great advice here!

[Nick Loy]

I used to do Fiberglass work on boats. Using real gelcoat to repair small area is an option. Using gel coat on large areas (outside a mold) is more than difficult. If you want a a complete bright shinny coat there are system (paints) like Imron that are designed to be new coatings ... contract for paint manufacture for their instructions for surface prep.

Frankly, unless you like to do lots of sanding with very fine grit, hire someone who does this professionally.

[Jay H]

From a voice of experience, two pieces of advice:
1. Listen to your wife. ( I have always found this to be good advice for many an occasion!)
2. Only sand through the gel-coat where you need to repair damaged fiberglass. The small holes and fractures in the gel-coat can be filled with epoxy (or some epoxy based auto-body fillers) and then sanded smooth again.

As many have pointed out, the gel-coat is the smooth outer finish of the trailer, you will never get the fiberglass layer smooth enough to look good under paint. Many of us have done this. I had damage to my shell that I had to repair and chose to paint the entire trailer rather than try to put a new gel-coat on.
It is also worth noting, the gel coat is what actually makes the shell of the trailer opaque. I have a section in the roof where there was a hole, I repaired the hole but did not have access to gel-coat and simply primered and painted over the new fiberglass. I then replaced the headliner inside the trailer and when I am inside on a sunny day that spot on the roof glows! The fiberglass alone is translucent...

TLDR - always do what your wife says!