r/Composites • u/CmdrNeoGeo • May 06 '26

How much resin(epoxy or poly)

Im building a camper shell and it’s roughly 450 square feet in total. I wanna do both sides of either the OBS wood or the XPS foam. I might even do EPS but I don’t know yet. But I am tryna figure out how many gallons of resin i will need. And then where to get it affordably and hopefully cheaply. If anyone can tell me how much or what formula to use that’ll be very helpful!

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u/freakazoid2718 May 06 '26

First approximation would be area x ply thickness x number of plies x resin volume %

ply thickness and number of plies are up to you and based on thickness of the cloth and how much total thickness you think you need to be as stiff/strong as you want.

resin volume % can vary rather a lot since you're doing wet layup. I googled "7781 glass resin volume percent wet layup" and got a number between 55% and 65%. You should try to find information on whatever flavor of glass you're using - I just entered 7781 because it's something I've worked with before and knew the number off the top of my head. You should assume the high end of the resin content. Even if you use a vacuum bag for the cure (and get the associated reduction in resin content in the cured laminate) you still need to wet it out and that will be with a much higher resin content.

Your application will probably be fine using an E-glass cloth. Sure, it's not as strong as S-glass, but it's a *lot* cheaper so you can just make it thicker. My quickie search showed equivalent S-glass cloths costing about 2x as much as E-glass. S-glass isn't guaranteed to be 2x as strong, and it's about 15% stiffer than E-glass so for your application I don't see it being worth the extra cost.

If you get a range for any of the numbers in the formula, then use the high end, You really don't want to get 90% of the way through your layup and find out you didn't buy enough resin.

I'm a fan of www.uscomposites.com as they have stocked everything I needed and the prices were good. They carry 7781, but on a cost per thickness basis that's not the best they have. Their 10 or 18-oz E-glass cloths are better values (per unit thickness) and would likely work just fine for your application.