How does one buld an FRP cabin door?

If you use epoxy just thicken it with microfibers and some silica and glue the core to the moldings. Glue a backing strip of premade laminate to the perimeter, the foam alone or your proposed putty is not enough to keep the two sides togheter. Then you can fill the remaining gap with epoxy based fairing compound and paint over it. Alternatively you could laminate a strip of fiberglass on the outside.

I tought you were using polyester and gelcoat for the moldings, using epoxy does not really make sense, you have to fair and paint the surface afterwards, negating the biggest benefit from having a mold. After the polyester is cured you remove the wax or PVA and can use thickened epoxy to bond the core to the skin. Epoxy bonds well to cured polyester, ther will be no problems, and you can use extruded polystyrene foam (XPS, the blue or pink insulation foam) for the core. PU foam (also used for insulation) works with with epoxy or polyester based resins.

 

Don't forget a way to screw in hinges..

 

My post was poorly written
Gel coat then poly resin with CSM to make faces in mold
faces epoxied to core with weighted pressure and filled where needed with white epxoy paste as poly resin will eat pink core
four SS 1.5x 5.5" hinges with #8-32 316 pasivated machine bolts and acorn nuts with loctite threadlock to fasten each hinge
So faces are thru bolted on one edge with hinge fasteners and by door lockset on the opposing edge

 

How thick is the skin?

You can't put a hinge bolt through a light frp skin and a xps core. It isn't strong enough. A good breeze one day or a swung hard door and the holes will open.

You really need some solid core there like coosa or timber even

 

I was planning on 4 layers of csm on both skins

 

It will still be too easy to crush with a bolt and then if you are light on bolt pressures; it will like to loosen and widen the holes. You really will be glad you cored it heavier. If you think the csm would not crush sans core; then go for it. Because this is not a core that does much.

 

I hear you about crushing the core. From experience I know the glass will be ok but I also would rest easier with a sheet of 1" close 20. Getting single sheets into Mexico is a hassle so I'll have to think on it after taking a closer look.at the frames and molds this morning. Thanks

 

Coosa 20

 

Well, you don't need a whole sheet. You need like 2 pieces 4" by 4" per 2" of core.

If you can find someone to sell you a 2'x4' panel; it avoids freight here US anyway. But just use some wood. If it rots; you just decore it and fill it with putty which you would be smart to do first...it is just a hardware backer

don't put the endgrains same as the screws or they won't hold anything

best practice is about a 3/8" overbore and fill woth thickened resin; then drill for the hinges and the wood is never even touched..

 

My 2 doors measure 69 x 24" = one 4x8' sheet of $350 1" Coosa to do both. I do not have any wood in boat, so I won't lose marketing point over a door.

Poly urethane as I understand is around 1/3 the price of coosa, just not sure if its stiff enough.

Any other core that is stiff and reasonable?? Kayboard looks to have the same pricing as coosa.

 

You don't need high density core in the whole door. You only need high density inserts where the hardware goes, to be able to screw into it. A piece of coosa would be enough for all the doors you will ever make since you have to cut it small pieces. The simple and cheap way is to just take all your fiberglass scraps from other laminations, mix them with resin and some filler and put it where the holes for the screws will be drilled. Problem solved, and marketing can say you have solid fiberglass inserts for fasteners. You should actually do this in every place a bolt or screw penetrates a cored panel.
Put some fabric on the doors, csm alone is good only for interior doors.
If this a production boat don't use XPS, you don't want the bad publicity.

 

You can metal bush the bolts as well with some stainless or aluminum tube, bit honestly, I prefer wood. You can also buy vinyl house trim or anything that won't crush. Vinyl house trim might be available in full inch.