Originally Posted by zstine
I want to through bolt a folding step to a FRP cabinet that has a good 1" gap behind it, then another FRP wall. So the hollow is like 22" x 20" x 1". The issue is the through bolt can't be tightened b/c the fiberglass
wall just flexes. I think I need something stronger than foam with the weight of a person standing on the step. And I have nearly a gallon of epoxy that's 3-4yrs old on-hand and would like to use up vice buying
foam, which I have no experience with using.
I wasn't thinking the pellets would absorb heat as much as spread out the bulk of epoxy to slow the reaction. I don't know if the pellets floating will be an issue? The small ones may stay put b/c the viscosity of epoxy. With the big ones, I may only use enough epoxy to cover and fill the voids between beads, and since I'm filling in stages, I'm not sure floating will be a problem..?
Without knowing the thickness of the fiberglass skins involved, & the footprint of the step's fasteners; making a proper recommendation is tough(er).
Because, if you're concerned about things flexing now, & or the strength of the skin, when adding such a step. Then the point loading forces imparted by said step, will cause surface (finish) cracking. Or worse; structural (laminate) cracking. Even if compression
tubes are added between the 2 skins. As the forces from the step won't be spread out over a sufficient area in order to prevent such.
Particularly as it sounds as if the outer skin is pretty thin to begin with.
Also, given that there's only 1" of room in between the two skins in question. Then adding blocks of any size in between them, inserted through small holes, so as to spread out the loading, isn't possible. At least not without removing part of, or all of, one of the skins.
As you cannot pivot/rotate a block of any size into position, inside of the 1" gap between the skins, described.
And as to pouring resin into such a space, as noted by others, along with the reasons why; it's in all likelyhood, a recipe for disaster.
Especially as, who knows where the poured resin will wind
up leaking to, unless you are 100%
certain, that the space in question is sealed.
In which case, you'll then have problems from the pressures created by offgassing during the cure. On top of possible/probable issues caused by heat during the resin's curing.
The simplest solution, is to carefully cut out one of the skins; completely or say 80% of it, out. And either add (bonded in place) substantial load spreader/compression blocks, for the step's fasteners & loads. And then to reinstall the skin.
Or to remove the skin in the area (or the majority there of), & fully fill the cavity with structural material. Followed by re-attaching (bonding) the removed skin.
The easiest, & most prudent, would be to do the latter. As, thus, that entire section of the transom(?) is now solid, & able to recieve other fasteners & fittings, at any point down the road, as well. Should you desire to do so.
Plus, the load from the step will be spread out over a much larger area. So that you shouldn't get any, or much, stress cracking. Including in the gelcoat(?) if the step is mounted properly.
-> Albeit, when mounting it, it couldn't hurt to add an oversized, external plate, in order to spread out any compression
& flexing loads. Even to the point of making this plate big enough to cover any "surgery", if you choose to do such on the outer skin of the area in question.
However, if you want to skip putting any external plate on the outside of things. And not to do any skin removal
on the outside. Then your best bet is to remove a piece of the inner skin which is 80% of the size of the area in question. Fill the gap between it & the outer skin with structural, non water
degradable, material. And then put the inner skin back in place, with structural taping/tabbing around it's edges.
As to the original idea of pouring resing into this space, though a small hole;
- You can add at most, 1cm (measured vertically) of resin at once, & then have to add the next layer while the first one is still in the "green" phase. To assure a proper bond between the two.
Adding any more than that & you're looking at it going exothermic.
- Using resin of that age, for something structural, is perhaps ill advised.
- Mixing it with foam bead, & the why's behind such being a bad idea has been covered by others.
- 1gal of resin is only enough to fill half of the described space... assuming that none leaks
out of it's edges, to wind
up who knows where.
You'll likely spend more time trying to fill this void, using some type of liquid/gel, epoxy filler, through a small hole, & wind up with results of questionable structuarl properties, overall. Than you would to simply open the area up, properly fill it (structurally), & then reglass/reseal things.
That, or simply add an oversized structural plate to the outer skin of the area in question (along with compression tubes). Be it; composite, G10, aluminium, or stainless.