Questions about filling in a couple 15" wide holes with FRP sheets please

Hmmm, I'm not sure we understand each other... How can I create a clean "cove" type fillet when the place I'll be running my popsicle stick along is so ragged (i.e. the bottom ragged edge of the overlap)? Please look at the photo here which shows the approximate area where the fillet will need to go between the two red lines. Its the top red line where the overlap is ragged that needs to be fair so I can run my popsicle stick along as a guide when applying the thickened resin. How can I do that? I was thinking of hot gluing a piece of wood below the ragged edge on to the flat surface of the motorhome and then fill in the gap between the overlaps ragged edge and the top edge of the wood with thickened resin. When the resin has cured I would remove the wood and have a straight smooth surface of hardened resin to run my popsicle stick along when I do the actual fillet. This should enable me to create a clean "coved" fillet that would require minimal sanding. It will be easy to pop the hot glued wood strip off the surface after the resin gap is cured but what can I use on the top edge of the wood so the resin won't stick to the wood?

 

The initial fillet will be a rough thing, done as best as you can with a mixing stick or whatever. The finish is treated differently, as this is part of the fairing process and a separate process, with different mixtures.

You could tape (or hot glue, if you could remove it without damage) a stick along the line you like to create to offer an edge to pull against, but you'll still need to finish by hand, where the tape trick will come in handy. If you use the stick, have it extend well past the ends of the repair area and of course parallel to it.

It would be nice if you do both at the same time, but the fillet material will be structural at first (really hard to sand, but strong), then a fairing compound (really easy to sand and shape) for the finish, under the topcoat.

 

Is the "tape trick" you mentioned below what you described in Post #15 where you said this?

"As to a fair line, the usual method is to fill and reinforce, so the repair is sound and treat the cosmetic portion separately. After applying the fairing compound, use a piece of tape to suggest the line you want and sand only to it from one side of the seam. Place another piece of tape on the other side, and work to this piece. You're sneaking up on the line so to speak, using the tape as a guide to keep you from sanding "over the top" of the line, you're trying to create."

If so, dang, I wish I could talk to you about that because I still don't understand.

 

Basically, you fill so it's just "proud" with fairing compound, most of which will be sanded away. The tape is applied right where you want your crease. You sand up to the tape on one side, then do the same on the other (you move the tape of course), until the you've knocked it down to where you want it. The tape is just a guide and keeps you from removing material over the crease. Send me an email and we can talk about it over the phone. I'm out of town for a week starting Tuesday, so tomorrow is it for several days.

 

Hey everyone... I had some questions and Paul ("PAR") was cool with me calling him on the phone and was super generous with his time in explaining a few things. He has a TON of experience with this stuff and seems like a good guy to hang out with and talk story whilst sucking a few brews. Anyway, thanks again Paul.

After rolling a few ideas around in my head I came up with what I think will be the best solution to clean up that ragged fiberglass seam. I'm going to hot glue a straight piece of wood above the seam and use it as a guide to run my router along so I can trim that nasty seam straight and clean. I'll use a straight bit set at about 1/8" deep. I really think that will do the trick with minimal hassles. I'll report back later how it went.

I wondered if the openings I glassed in from the inside with 3 layers of 6 oz E cloth and lots of thickened resin (milled fibers and cabosil) on the outside seams might telegraph the seam through after painting. Paul said it would and that I should sand a concavity along the seams with some gnarly 40 grit on my grinder and then lay a diagonally cut strip of 6 oz E cloth with a layer of matte on top of that.

Related to that here is my question...

What is the maximum size in inches that an opening can be that does NOT require that I glass a layer of 6 oz and matte to it to avoid telegraphing later? I'm asking because I yanked all the little clearance lights off ("clearance lights" are the little lights seen near the top of a motorhome located on all four sides). Each clearance light I pulled off left behind three holes (2 @ 3/16" in diameter and 1 @ 5/16" in diameter) each. Do you think those holes are small enough that I could get away with just filling them with thickened resin only...or would I need to sand those down and lay some cloth in them?

Thanks again.

 

I've been using a filler I made out of polyester resin, cabosil and 1/64" milled fibers for certain things on this project where I needed strength but that stuff is reeeeeeally hard to sand. Can anyone suggest what I should use to make a polyester resin based filler that can go on a vertical surface and is relatively easy to sand?

Also, can any polyester resin based filler be applied directly over a lightly sanded polyester gel coat?

 

Steve, those little holes will easily fill with the filler of choice (polyester or epoxy). If terribly worried about them "coming back", taper the edges a bit, like a way over size counter sink would, and fill with thickened goo, employing micro fibers, to improve adhesion and strength across the gap.

Any size gap can be filled with just filler, without any print through. Eventually, you run into a point of diminishing returns, where you need so much thickened goo that it can't bridge the gap, without something supporting it, from falling through. You can support it with anything temporary, until the stickum cures, at which point the temporary support is meaningless. I've used tape, cardboard, tin foil, you name it to perform this task. At some point, you have to decide if a big hunk of thickened goo is sufficient, in terms of strength. I arbitrarily set this at 1/2" gaps or less. If the gap is more than this, you usually need underlying support, so a piece of fabric can kill two birds with one hardened stone of cured goo.

Print through is when anything (usually the fabric weave) comes back through the hardened goo, once it's fully cured. This could be surface irregularities or just the fabric weave. To prevent this, a layer of mat (1 ounce or less) can be used as the last, outer layer of the laminate. This will prevent weave print through. Conversely, you can just fill and fair the surface after print through shows up. A blocking primer will do this with finish fabric weaves, but you'll need filler if the fabric is heavier than 6 ounces.

Yes, you can apply filler over gel coat though "lightly sanded" will likely mean it'll fall off if there's much vibration associated with this piece (powerboats especially). For any polyester product to stick well to polyester (like gel coat) it needs a substantial "tooth" to mechanically "key" the bond to the surface. This means an aggressive scratch must me used, or it'll just fall off.