Sanding rough interior of fiberglass hull

[SaltyBlonde]

Hey guys, I’ve torn out all the original carpet that used to line the fiberglass hull and will be using some alcohol and a plastic scraper to get the glue and remaining padding and what not off there.

I bought a square finishing sander and some 80 grit, 120 grit, 220 grit and 400 grit sandpaper to try and smooth out the hull for a cleaner look before I cover it in marine primer and a good topside paint.

My concern is that sanding the fiberglass may weaken it, and wanted to know if anyone had experience with it and whether or not it’s worth the investment of time to go over it with some faring compound afterwards before I primer it.

I’d like a professional clean look, but don’t want to compromise the structural integrity of the hull either. I don’t intend to put any carpet back up but rather leave it nice and bright white for a clean finish. Any thoughts?

Thanks much in advance! Appreciate your time for throwing me some advice.

[TrentePieds]

Quote: "... I’ve torn out all the original carpet that used to line the fiberglass hull and will be using some alcohol and a plastic scraper to get the glue and remaining padding and what not off there...."

Do you mean carpet (or perhaps a carpet-like material) that was used to cover the inside of the actual fibreglass "lay-up" of the hull? Are the surfaces you wish to refinish compound curved?

TrentePieds

[Sparx]

Do NOT sand into the fiberglass cloth. Doing so will never result in a smooth finish. If there is an odd fibre sticking up here and there, ok knock the top off these off with the sander Do use the sander with 80 grit to rough up the resin surface over the fiberglass in preparation for painting on a high build primer.

After 4 to 6 coats of primer sand the primer with progressively finer sand papers to 200 grit or finer. Then paint with a good quality acrylic latex.

[SaltyBlonde]

Originally Posted by TrentePieds

Quote: "... I’ve torn out all the original carpet that used to line the fiberglass hull and will be using some alcohol and a plastic scraper to get the glue and remaining padding and what not off there...."

Do you mean carpet (or perhaps a carpet-like material) that was used to cover the inside of the actual fibreglass "lay-up" of the hull? Are the surfaces you wish to refinish compound curved?

TrentePieds

It was a carpet like material, very thin. Likely a sound deadener and an insulator, but ugly as hell.
It’s the interior of a catamaran hull, so it’s definitely curved in some areas.

[SaltyBlonde]

@Sparx
Ok that makes a lot of sense. Thanks!! Any primers you recommend that may be thicker than others?

[wingless]

Welcome to the forum.

When I was upgrading my vehicle headliner I discovered the headliner board was compressed fiberglass. That worked fine w/ the OEM padded fabric, but I was changing to an unpadded suede fabric. That change would have made all the surface defects visible on the finished result.

My solution was to glass over the pressed fiberglass, then mix fillers into the West System epoxy and squeegee across the surface. After it hardened I gave it a light vacuum sanding and the result was smooth, smooth, smooth.

It might also work in this application to squeegee West System w/ filler onto the surface to gain a smooth surface.

The desired top surface treatment could then be applied.

[barnakiel]

I never sand interior hulls. It tends to expose the glass. You end up all itchy.



b.

[SaltyBlonde]

@Wingless
Wow, I love the idea of that. It sounds like a lot more work but I like the idea of it reinforcing the hull a bit too and having a smooth smooth finish. I'll be doing this for all three berths and potentially the head too. So it's definitely going to mean a lot more elbow grease, but on the bright side, it will be a better finished product.

Appreciate the idea and the pics too! Thanks so much!!

[Chotu]

Originally Posted by SaltyBlonde

Hey guys, I’ve torn out all the original carpet that used to line the fiberglass hull and will be using some alcohol and a plastic scraper to get the glue and remaining padding and what not off there.

I bought a square finishing sander and some 80 grit, 120 grit, 220 grit and 400 grit sandpaper to try and smooth out the hull for a cleaner look before I cover it in marine primer and a good topside paint.

My concern is that sanding the fiberglass may weaken it, and wanted to know if anyone had experience with it and whether or not it’s worth the investment of time to go over it with some faring compound afterwards before I primer it.

I’d like a professional clean look, but don’t want to compromise the structural integrity of the hull either. I don’t intend to put any carpet back up but rather leave it nice and bright white for a clean finish. Any thoughts?

Thanks much in advance! Appreciate your time for throwing me some advice.

Yeah, that’s not what you do. Do not sand the Fiberglass. That is the structure of your boat. What you want to do is build it up with a faring compound and sand the fairing compound. Not the fiberglass. It’ll be a lot easier, your boat will not fall apart, and you won’t have Fiberglass itch when you are done.

Think of it like doing sheet rock. It’s not that much different. You put the mud spackle above the bumps and you sand the mud spackle to make a smooth surface. Same idea.

[TrentePieds]

Hm...

Yes, as I suspected :-) Cat or mono makes no diff. What is significant is that you have compound curves to cope with. Because of that, the pad sander you mention is not really up to the job since its base, while slightly padded, is not able to conform to a curvature that is constantly changing as you move the sander. In consequence you'll get "hard spots", and you run a real danger with the coarser papers, the 80 and even the 100, of cutting through the surface layer of the lay-up and exposing the strands of glass filament within it, which you don't want to do.

Energetic sanding with the paper laid around big, thick sponge, starting with 60 and moving up the number scale as the work progresses, may be a better bet because you'll be able to feel what the paper is doing under your hand. Not too much pressure, but lots of back'n'forth with a change of paper whenever it fills with the nastiness of the adhesive the "liner" will have left behind when you stripped it. It'll be a heck of a job, but if you are up for it... :-)!

Doing this kind of work, there is merit, at each step of the process, in visually defining an area of about 18" by 18" and concluding the work on that little area before you move on to the next one.

At some stage you will then get to a point where you can "spackle" the surface, i.e. fill it with a compound formulated for the purpose, and deliberately made soft enuff that it sands readily. The game then becomes spackle, sand, spackle, sand - on and on till you have a perfectly smooth, paintable surface.

In the old days, you would have used a few coats of primer, sanding between coats with ever finer grades of grit, just as Sparx sez, and then topped it off with a high quality (and expensive) marine enamel. Done right, you would then have a finish that in the words of MySaintedFather would "shine like catshit in the moonlight".

But you can't get marine enamel any more, cos some government agency or other decided that paints based on lead and oils aren't "on" any more.

I don't know at all what the chemistry is, but I've just used an "enamel" furnished by a garden variety paint supplier to refurbish a badly neglected and damaged townhouse. I was impressed! When I began, the paint felt absolutely weird on the brush, but as I learned its foibles, I came to like it. The "drag" on the brush was enormous, but the self-levelling was superb even on vertical surfaces. The coverage however, was a bit slight, and even with a good undercoat of primer, I suspect that for what you want to do, two coats with a rubbing down in between would be required. I haven't needed to test this, but I think this particular paint might need as much as a week to cure before you rub it down in preparation for the second gloss coat

Time to roll up your sleeves!

Let's know how you make out. We're here to learn from each other :-)

All the best

TP

[smj]

The Gemini has a solid glass layup that is pretty thin, last thing I would want to do is take away any strength by inadvertently sanding into the structure.

[Sparx]

Ok lots of good ideas flowing here.

For primer on well cured resin on a boat interior i think any acrylic primer will work. It does not need to be "marine".

This might work TotalBoat Topside Paint Primer

Talking to the team at Total Boat is a good place to start. Also visit your local Sherwin Williams store.

[Fore and Aft]

Salty Blonde do yourself a favour and buy some plastic drop sheets and masking tape. Then seal of any area's of the boat you are not going to sand and paint. It makes the clean up so much easier. It's also nice to get a fan and open a hatch near you so some of the dust blows outside.
Cheers

[TrentePieds]

Fore and Aft is absolutely right, and on that topic, which is so obvious to those of us who've "been there" is so obvious that we are apt to forget to say it: DO NOT expose your lungs to any of the "stuff" that goes flying through the air doing this sort of work! Wear a suitable respirator! Mind you eyes also. Filamentous glass, once worked loose from its binding medium, is like little javelins! Wear long sleeves and tape the cuffs tightly.

Onwards! :-)

TP

[Time2Go]

Hi Salty,
It sounds like you want to “Fair” the inside
surfaces. Get ready for a tremendous amount
of sanding,
Check out “Sail Life” on youtube
could be an eye opener.
I’d investigate ways to laminate some type
of material onto the surfaces
could save you a boatload of time
Cheers
Neil