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I am mentally going through the steps in lengthening the engine box hatch on my B25.

Here is a shot of the cover. My initial plan is to split the cover on the red line and basically pull the two pcs apart by 12 inches and glass back together filling the void. I have in my mind how I'll hold the pcs, at the correct distance apart and basically make a mold to lay the new glass in.

The box is cored with 3/4 corecell. Do I need to remove all of the core or would removing about 2 inches on either side of the new joint be sufficient. I plan on coreing the 'new' glass with same corecell.

Thanks in advance.

Ian.

Ian,

Make your mold before cutting the box. Use the box as a plug and cross support the bottom opening to minimize any warpage with the box standing free. Several misted on coats of PVA will do a good job of mold releasing the box.

Make the fiberglass skin no thicker than 1/8" and build up the thickness of the mold wall with 3/4" plywood and thickened resin as a bed in material. Add any necessary gussets and angle supports to keep the mold rigid after removing from the box.

Your first layer will be gelcoat, followed by very light weight glass. That can be veil, 3/4 oz mat, or 1 - 2 oz 0/90º cloth. Let those two layers completely cure over night and follow up with a layer of 1 1/2 oz mat. That schedule will be about 1/16" thick. The thinner you keep the fiberglass thickness on the mold, the less shrinkage you'll have.

Let your mold completely cure for several days to get rid of all out gassing of the resin. Any out gassing that might occur when an actual part is being laid up will result in the part sticking to the mold, regardless of mold release added.

After the mold is no longer green and doesn't have much of a fresh fiberglass smell, apply at least 3 or 4 applications of mold release.

I prefer to let my molds age a minimum of 5 days before trying to pull the first part. Just as in making the mold over the box, your first two layers will be gelcoat and very light weight glass. This is your skin coat. Let it cure until hard, (2 hrs or so), and finish the layup to the desired thickness. This will assure you of a minimal amount of print through of the glass fibers in the gelcoat.

You don't need to remove all of the Corecell, maybe just an inch or two from each side. Leave the new section in the mold until you've cut and configured the box.

When the box is ready, remove the part from the mold, cut it to your desired dimensions, and with the box upside down as you have it in the image above, fit the part in place. Best to have everything down on a flat surface, preferably a cement slab.

Carefully fit the three pieces and using clamps, weights, battens and what ever necessary, dry fit it all into place. Using small tabs of glass cloth where ever there's a space between your jigging, tab the parts together and let it harden a few hours. Remove the jigging and run a full course of tabbing across the seams. Cut your new foam core sections and bed them in with thickened resin. Add the full fiberglass layer to cover the core.

I'd allow that to cure over night before turning the box back over.

Using a small abrasive wheel undercut the exterior seams down about half way through the glass. In these depressions, lay 1 1/2 oz mat, but stay 1/16" - 1/8" below the finished gelcoat surface. After that cures cover with gelcoat, slightly thicker than finished surface level to allow for shrinkage. Let it cure for 24 hours before sanding to finish level.

Br,

Patrick

Thank You!

It looks like I'm heading back to the fiberglass supply store before too long.

This box has been primed with epoxy primer had has some pretty good runs in it. I'll sand these as smooth as possibly before trying to pull a mold off.

If this will be primed and painted later, should I skip the gelcoat when I make the part?

Thanks again!

Ian.

I like to use gelcoat, even if very thin coat as my first layer. That will keep you from having pinhole bubles on the surface of the part. All fiberglass will trap fine bubbles, which when sanded cause pinholes. A layer of straight resin, allowed to move just past gel stage before adding the skin glass will accomplish the same thing. Gelcoat just leaves a very nice base for future paint and is more thixothropic than straight resin.

thixothropic??

randall wrote:thixothropic??

Actually, I mispelled it...

Thixotropic: Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.

Cabosil, (colloidal silicate). is the principle material used as a resin additive to create a thixotropic mix that resists running, sagging, and also aids in bonding.

thanks....thats a new one to me.

alex...ill take scientific terms for 2000.

Holy crap......I think I must be Thixotropic.....is there a pill you can take for that???

Harry, I think a bowl of Clam chowder and a dozen clam cakes relieves you of being Thixoproxic.....if that doesnt work a couple of cold ones may...BH