Surface building after gel-coat removal

[MEJETSKI2000]

I've removed all the gelcoat from the topsides of my FJ and I'm adding some mat to reinforce the seating areas where the gelcoat has stress cracked. How many layers of mat does it require to equal the durability that the gelcoat layer offered ? I'd say it was maybe .050" thick originally. I'm going to re-apply gelcoat after the repairs are completed.

The gelcoat provided cosmetics and some water resistance. The reason it cracked is that what was under it cracked. Matt offers not much strength, for this, I would recommend grinding down to sound substrate, building up more than the previous laminate with cloth ( it's not the strongest glass but you get quite a high glass/resin ratio and it is easy to lay smoothly), then coating.

[Dan H]

The gelcoat doesn't add any strength. The gelcoat cracked because the fiberglass laminate under it flexed, or the gelcoat was applied too thick in the first place. .050" seems pretty thick for gelcoat. .025" is even kinda thick.
Mat is not used for structural reinforcement but is more used to hide print through and increase the surface area of contact between the courses of roving woven.
Gelcoat is sprayed so thin these days you can see the mat through the gelcoat so thicker is not better. Stop by a RV dealer and look at the fiberglass camper wall real close. You can see the mat through the gel.

If you're going to add strength, you could add a course of 12oz stitch mat cloth and epoxy. That alone will add a lot of strength. Fill the weave. Make sure you cure the epoxy real well using heat, scrub off the amine and sand it. Spray the gelcoat over that.
Maybe the experts will correct me.
That's what I'd do.


http://www.fiberglassindustries.com/knitmat.htm

[MEJETSKI2000]

It's starting to sound like I need to go the mat and cloth route. I was hoping to avoid using cloth to keet the weight down, but I guess that's not going to work.

I thought gelcoating over epoxy is a no-no....do you mean resin instead ?

I'm also removing the old gelcoat from the exterior of the hull, can I build the surface up with multiple coats of epoxy (using Epiglass) to replace the gelcoat before I apply the Interlux primer and exterior paint ?

[pescaloco]

Originally Posted by MEJETSKI2000

It's starting to sound like I need to go the mat and cloth route. I was hoping to avoid using cloth to keet the weight down, but I guess that's not going to work.

I thought gelcoating over epoxy is a no-no....do you mean resin instead ?

I'm also removing the old gelcoat from the exterior of the hull, can I build the surface up with multiple coats of epoxy (using Epiglass) to replace the gelcoat before I apply the Interlux primer and exterior paint ?

Sorry to answer for the other guy

Yes gelcoat over epoxy is a big no no - try some vinylester resin and cloth then gelcoat

[Dan H]

After looking at what an Fj is, I think adding fiberglass will add too much weight. If it were mine, I would not have worried about the cracks. However, that is water under the bridge. I would just spray gelcoat back on and it will probably last several years before cracking again if you don’t spray it too thick.
If you’re dead set on adding strength, add as little as possible. Try 4oz cloth and epoxy, then spray gelcoat over that.
Yes I am one of those guys that sprays gelcoat over epoxy.
Here is a video from West Systems on how to do just that. They also have test results they ran applying gelcoat over epoxy for use in underwater applications. That info is on their web site.
Do not use mat with epoxy because the mat is coated with a binder that is only compatible with polyester resin. I heard there is mat out there for epoxy but I don’t what it is.
That’s what I’d do.
Here is a photo of a repair I made spraying gelcoat over epoxy. It’s been two years now and looks perfect.

Gelcoat over epoxyYoutube Video

The repair is right between the handholes and up over the gunnel.

[themanshed]

Touch up work gelcoat is ok it can even be rolled or brushed on if you do not have spray equipment. To redo the whole boat paint is better and less work. I've done it both ways. The key to weight is not the cloth but the amount of resin or epoxy. The rule is after wet out the more resin the less strength and more weight.

Use cloth - mat has no strength. If you are really worried about weight vacuum bag the area pretty simple and a shop vac can be used for a small area. The next best thing is to get 3M masking plastic, make sure lettering is facing up, lay it on top of the lay-up and squeegee the extra resin and air out it will leave a nice smooth surface. Last point - don’t use bondo as a filler for fairing, best to use thickened epoxy or resin it will last longer and not swell.

[Herman]

About applying gelcoat or topcoat over epoxy: Please do make tests before you do the real thing.

I sell epoxy resins from which I know that won't work. (polyester will not cure).
I sell other epoxy resins from which I know that it will work.

About mat for epoxy: There is stitched glass matt. Some non-crimp-fabric producers use it. Another solution that was once given to me is add about 10% styrene into the epoxy. Can not recommend it though...

[MEJETSKI2000]

I was going to use a roller for the gelcoat, as I haven't got any spray gear. I try to use as little filler as possible. I have no clue how to vacuum bag or anything like that I'm afraid. I'm thinking some 4" tape in the crease where the gelcoat cracked (and repairing the cracked substrate) should add some strength w/o too much extra weight. The boat's capacity is 600 lbs, incidentally. I intend to add a layer of mat and lightweight cloth on the deck to reinforce the single layer of roving and textured resin, which is too flimsy for what I need this boat to do (not racing). Dan, thanks for the video link for the epoxy info. I've been reading up on non-blushing epoxies and I think some MAS will work well for the bottom-side, along with Interlux 2000E primer before top-coating. Jury's still out on the color however......