Stock build up/re-profile - MarineTex or other products for the build up?

[Rich S]

I'm helping a friend re-profile of a fiberglass stock. It is a basic fiberglass stock with 3" flat forearm and the butt has drop. He wants it converted to a flat butt stock. The issue is the shell of the stock is relatively thin and will not allow milling the profiles he wants. He plans to build it up then I will machine it to get the profile/fit he wants for his bag. I've taken a few cuts to give him a flat bottom to provide a base/datum to work with for the build up. Now we need to decide what to build it up with.

It looks like MarineTex may work for the build up. Their information says it can be built up to 1 1/2" thick. I'm guessing we will be ~1/4". For those that have used MarineTex do you think it is durable enough for the stock exterior and is it machinable? I'm hoping to set it up in the Bridgeport and use an extended endmill like I have a couple of laminated stocks. Any recommendation on other products?

Thanks
Rich

 

[EWaugh]

I remember a few years ago I had a similar situation with a plastic stock that needed a recoil pad. I profiled a piece of hardwood to fit inside the stock and seated it about ¼” in, bed it and the gap with devcon putty. It worked great for me. And I don’t see why you couldn’t accomplish the same with just using Marine Tex.

 

[Mozella]

They say when you're a hammer, everything looks like a nail. I'm an experienced composite designer/fabricator and here's what I would consider. Any kind of filled epoxy, unless properly supported, is prone to chipping or you might even lose a large chunk if the gun is dropped. Plus, it may be hard to hide the line where the patch joins the original stock.

If you have a West Marine or other marine hardware outlet nearby, purchase some West System (no relation to the store) epoxy and hardener. It comes in small quantities. Other high quality, unfilled, two-part epoxy is OK too but avoid any 5 min or 1 hour epoxy.

You will need some filler and some thickener to mix with the epoxy after you mix the resin and hardener together. I recommend glass bubbles or some similar light weight filler and some Cabosil (colloidal silica) to thicken the mix. You don't need much. Stir the unthickened epoxy very well first, then add the filler and mix carefully. Make it thick enough so that it doesn't run.

Scratch sand the stock VERY thoroughly and apply the thickened epoxy. Let it cure and, starting with a Surform tool (cheese grater style shaping tool) shape the stock. You can do this by hand in less time than it takes you to set up your milling machine. Finish with 80 grit sandpaper. Then carefully lay up one or two layers of light weight fiberglass cloth, 4oz/yd if you can get it, 6oz if you can't. Wet it out with unthickened epoxy and squeegee out as much epoxy as possible without over doing it. Watch a YouTube video.

When it's nearly cured, brush on another coat so you'll have something to sand without cutting the cloth. After the first sanding, brush on another coat of unthickened epoxy and sand. Do this three times or as necessary to achieve a nice finish. This takes several steps, but when you're done you'll have a stock without thick gobs of unsupported filler which are prone to fail. It will be similar to the construction of a McMillan stock; i.e. fiberglass on the outside with a thickened epoxy filler on the inside.

Now if you say, "Dang, that's way too much work", then consider this routine as a second choice. Start as before but use some chopped cotton ("Flox" or "Flocked Cotten") or some milled fiberglass strands (good, but they make you itch) when you thicken the initial batch of epoxy. By using some sort of stranded material to thicken the epoxy, your filler will be much stronger than ordinary MarineTex. You can eliminate the fiberglass shell and reduce the sanding to a reasonable amount, but it won't be quite as durable as doing it the right way.

 

[lloydx2]

USC Dura Glass. Strong, Lightweight, Heavy fill. Does not shrink. Just add cream hardener and go.
You will want to skim coat final app with Glazing putty (pin holes). Amazon has it for $19 a quart. I have used
it to build up cheek pieces on McMillan stocks. Adhesion is outstanding.