POD: Homebuilt foam core fiberglass skin pop-up camper build thread

[windsock]

I watched some kayak builds on YouTube the other day and they were using these long boards for finishing off their fibreglassed externals. Looked really good and produced a beautiful finish on a curved surface. I was thinking of maybe using one on my build. I don't want to highjack pods8 thread but how flexible are the boards? Would they work as well on a flat surface or would I be better off with just a random orbital?

Sorry if deemed a hijack in response. The beauty of the long boards for sanding is that you get a large area covered evenly versus a small area possibly unevenly as with hand sanding. It may seem like an even job is done with a small sander but once the gloss coat goes on any minor hump or hollow is exaggerated. May not matter much in some quarters but on yachts where the beauty is in the lines, it mattered much. I guess the same can be said for camper lines too. On surfaces meant to be flat in apperance, humps and hollows can easily be avoided using a long board that is rigid enough to absorb the pressure applied by the arms to do the job. For curved surfaces i would use ply that is thick enough to 'just' take the shape of the curve under pressure. For flat surfaces I would use timber with very little flex at all. The sanding action is important too. Always keep the strokes moving around and never sand in one place too long. Spot sand for obvious bumps but use the long board to get large areas flat (or curved areas curved smoothly whatever is required).

 

[Heifer Boy]

Thanks for the tips windsock. That sounds great so I think I will make one up and give it a try.

Back to you pods8...

 

[pods8]

It was a relevant sort of hijack and has me thinking a tad more.

I'm wondering if trying out a cheapie inline air sander: $30 harbor freight would be worthwhile or not. I've also heard of folks converting over a foam pad polisher to a fairing sander by putting a piece of thin plywood over the foam and then sticking the sandpaper to that... Hmm got a couple days to stew/obtain supplies.

 

[r_w]

Those sanders work, but you need to make sure they don't blow oily exhaust onto your work. Some come with exhaust filters, some don't. Those that don't can be retrofitted with some gauze and duct tape.

ETA: the cheap ones take a LOT of air to run--that one is 5 CFM @ 90. If you don't have a huge compressor, it is worth the money to get a better brand with a more efficient air motor.

 

[pods8]

Those sanders work, but you need to make sure they don't blow oily exhaust onto your work. Some come with exhaust filters, some don't. Those that don't can be retrofitted with some gauze and duct tape.

ETA: the cheap ones take a LOT of air to run--that one is 5 CFM @ 90. If you don't have a huge compressor, it is worth the money to get a better brand with a more efficient air motor.

Very true, I've got a 20gal compressor but would need to double check the continuous CFM rating, that might be pushing it. Edit: I just checked it out, it is a 25gal 2hp unit rated for 6.8cfm @ 90psi so I've got a bit more air to work with than I expected.

The sandpaper/plywood mod on a buffer might be something more to check into as well since amps are easier to supply than CFM.

 

[stomperxj]

Subscribed.... Great progress and awesome info

 

[pods8]

Those test pieces on this foamular 600 foam (verse the foamular 250 I was previously experimenting with) had sufficiently cured and I subjected them to the hammer test. In reality this is asking for much more impact strength than really needed, I mean if you hit an aluminum sided camper that didn't have foam bonded on the backside (ie like my FWC with fiberglass insulation) this hard with a hammer you'd likely punch into it. But its something to shoot for.

Below is the photo of the 4 test pieces with a HARD hammer hit to the center, all performed in a manner I'd generally be comfortable with using. The 2ply piece experiences more deformation but still fared okay. Three plys did well, yes there is cracking in the matrix but not a total failure. The 2 plys over 1/8" plywood held up the best as could be expected but its a definite weight trade off.

I know the questions will inevitably fly on weight, cost, etc. so I'll shine light on the information I put together to help decide on the path for the shell construction. I weighed each of those sample pieces as I assembled them. The results below show the component weights. I had to make an assumed correction for the glass weight since the pieces are cut oversized and then trimmed after the epoxy has set up a bit. I used the published weight figures for the cloth to make the correction. I was expecting a realistic glass ratio of near 50% so the 55% is appealing and I believe realistic for these flat pieces.

Below that is a summary matrix of which type of ply schedule on interior and exterior verse the weight. This is based on a rough estimate of 400ft^2 for my shell. This is the weight before the weave is filled/faired and painted. There is no allowance for windows, fittings, etc, etc. Just gives a ballpark weight for foam panel construction on its own. Also no allowance for the wood inserts I'm doing in my build but that would be the same no matter with ply schedule I went with.

And finally the cost of just the foam, glass, and epoxy to make those shell configurations. Again no allowance for shipping, tax, application supplies, etc. These are just rough decision guidance calculations.

Enough with the caveats here's the numbers:

 

[windsock]

Excellent post pods8. I don't recall reading what you do for a living but I would hazard a guess at scientific or engineering. Your approach equates to a high level of analysis and I appreciate you putting it up here for us.

I started to wonder about the hammer test after you wrote up on it above. Is it possible to do a crush test under known pressure conditions? That is use a mechanical press and say an area of pressure of twice to three times the area of the hammer head. It would be interesting to assess the kind of pressure your coating preparations can cope with before deformation and then penetration. I would think with a larger area the results would be surprisingly good given the hammer test results.

When I made and lived in a large truck and camper (many years ago now) I made the shell out of steel frame and aluminium skin mostly. In the high impact areas, (i.e., lower sides and front quarters of the cab-over), I used painted plywood. If it was damaged (and it was frequently) it was a simple case of fitting new ply over the damaged area until I was next able to repair with a new ply section. Worked well and I was wondering given the ability of the ply in your test to absorb high impact, maybe if concerned about damage, have a ply layer in the potential high impact areas only and glass-over the rest.

Keep up the great work. Liking it a lot! :victory:

PS (edit). I hope you are keeping all your test pieces. All joined together after the build completion in chronological order and framed would make a nice conversation piece in the workshop...

 

[pods8]

I don't recall that I've ever said what I do. I tend to want to let posts ride on their own merit (or get critiqued accordingly) without as much weight riding on the background of the poster. You're barking up the right tree though, however much of this building approach is new to me and has nothing to do with my professional knowledge set, I've just used my research/tinkering skill set to bring me up to the point I'm at on foam core/composite type work. Lots of reading on composite airplane building and some boat stuff. So with that I'll say this disclaimer: Don't take anything in my thread as professional advice and you're on your own liability.

I don't have the resources to do a known load compression test, if someone with that stuff wanted a test piece made up to play around with I can make one up from scraps as I get time. However from a more practical look, anything that can take the rapid impact of a hammer hit, which has both a lot of load but also rapid application of it, decently well should survive most situations I'll throw at it. The main section that I'll put the most critical eye on is the rails on the cab over section, those will require a build up of multiple plys to support the cantilever.

I've got a pile of the pieces that I'm made and beat up, I don't have any real intention of holding onto them long term, they're just around for the meantime if I need to interrogate something on them further. After that I'll need the space! :coffeedrink:



Tiny update: I picked up one of those $30 inline sanders from HF and ran it for a little while, it runs fine on my compressor and I think it'll work nice for fairing work but even with 36 grit it was slower going knocking down elevation differences. So in the interest of time I think I'm going to pick up a cheap hand held belt sander to do the rough knock down work.

 

[Heifer Boy]

Tiny update: I picked up one of those $30 inline sanders from HF and ran it for a little while, it runs fine on my compressor and I think it'll work nice for fairing work but even with 36 grit it was slower going knocking down elevation differences. So in the interest of time I think I'm going to pick up a cheap hand held belt sander to do the rough knock down work.

I succumbed the other day bought a belt sander too. It's a very handy tool but pretty brutal if you're not careful. I had rented one years ago for a quick job but figured I could justify buying on this time. It was just a $69 cheapy which is a day hire for one so I'm ahead already.

I also made up one of the 'idiot' long boards windsock was talking about the other day similar to the ones in his photo. I made up a stiff on on a 15mm ply base becasue I'm just sanding large flat surfaces. A quick play around and I love it. What a great tool!! I'm actually looking foward to the final external coats now.

 

[pods8]

Update 3 (27hrs in): A $30 harbor freight inline sander and $30 harbor freight belt sander were good investments on this project (note if I know I'm going to use a tool a bunch I'll splurge for quality but these things will likely get used/abused on this project and not have too much other use so the HF stuff should fit the bill well). 80grit on the belt sander along the wood areas to knock that all flush and then 36grit on the inline to fair out the foam. My compressor doesn't quite keep up for continuous use of the inline but it only slowly looses ground so I can get a good chunk of continuous sanding before I need to hold off and let it fully catch up before hitting things again. I think this will come in handy while doing fairing work after the glassing as well.

1/8" ply was epoxied down top and bottom with narrow crown staples & some pavers to hold things in place in the meantime. I had one mild bonehead move in that I drew out where the wood was on the upper 1/8" ply for easier reference later but then proceeded to staple down the first sheet mirrored in the wrong direction (front to back) due to the core being in a different position after I had flipped it from doing the bottom and me not thinking about it till after some staples had left the gun. Easy enough to just transfer the marks later though so no real harm other than a bit of wasted time.

The 1/8" ply is slightly oversized right now. I'll use a flush trim router bit along the top edge and a 1/2" radius bit along the bottom foam edge to trim it up later.

 

[Heifer Boy]

Good work pods8. Can I borrow some of your paving slabs? A couple of those would have come in handy a while back...

 

[Billhilly]

Looking great.

 

[pods8]

Good work pods8. Can I borrow some of your paving slabs? A couple of those would have come in handy a while back...

I was feeling bad last fall when I didn't get around to laying these into place for a larger landing outside my garage side door, however right about now having a handy pile of concrete blocks is rather nice. :coffeedrink:

If I was doing the whole construction with plywood skins I'd probably think about a vacuum pump instead though.

 

[Lynn]

Pardon me for feeding the hijck, but I've been wondering if an old waterbed matress would be good for weighting a large panel. Whata ya think?