I'm rather struggling with this so another question.
Caravan, alloy outer skin which is damaged and I need to repair it with car body filler.
The problem is that car body filler is reputed to attack polystyrene which is just behind the alloy skin and used for insulation. So I need something to separate the two.
Your suggestions please.
1) test attack theory first ?- whilst there test for attack on thick polythene? 2) thick polythene tucked well in beyond the damaged area?
NB I mix up cbf on a supermarket plastic "worktop saver" - thin flexible type usually translucent (& coloured)..
Jim K
Hardboard/ aluminium foil? A blob of filler on the edge will hold it in place while you do the rest
Almost anyth8ing.
epoxy does NOT attack polystryrene by the way so you could use epocy filler instead.
Other possibles include simply slipping a bit of newspaper or plastic bag in the gash and pushing it about a bit to protect the polystyrene. Even painting it with PVA and sticking brown paper on will work.
I guess sliding something like ordinary cooking foil or a plastic bag into the gap would be tricky. Ex takeaway container ali foil, Ex rigid plastic food container, cardboard from cerial packet?
How big is the hole/damage? Will CBF stick well enough/be thick enough to withstand the inevitable flexing of a caravan body?
Really, I know some car body filler gets hot as it cures, is this what you mean.
Brian
Thanks, not so big an issue as I thought then. The actual exposed polystyrene is about 2" x 2", though the damaged area is larger.
I was thinking along the lines of even the fumes from the filler attacking the poly.
Dave Liquorice formulated the question :
I don't anticipate it flexing much at all, where it is located.
Brian Gaff used his keyboard to write :
No, it is the chemical hardener in the filler which attacks the polystyrene, it basically turns it into slime according to what I have read. I just needed some advice on whether just keeping the two separated was enough, or whether one needed to be sealed from the other.
I saw a video of such a repair and they used some substance to seal over the polystyrene, before applying the filler.
Not quite true. Car body filler is mostly styrene monomer with an inert filler, the hardner triggers polymerisation to polystyrene. The problem is that the polystyrene foam insulation is somewhat soluble in the liquid styrene but this should not cause real problems - particularly once it is cured.
If you are worried about it getting hot, don't overdose it with the hardner & it will cure more slowly. It will stick to polystyrene well as it is all the same thing really but I guess might cause some of the foam to collapse.
If a cavity forms in the insulation could it be injected with polyurathane foam?
HTH
Chris K
visions of caravans with bulging "hernias" on the M5 ;>))
Jim K
No brian. one of the components of polyester resin is a sort of styrene in a solvent. It does SLIGHTLY attack styrene.
But any physical barrier - even a layer of PVA - is enough for it to be a non issue.
No. i've spray painted expanded polystyrene with cellulose paints. After covering it with brown paper soaked in PVA. one pinhole caused a LOT of damage..but the rest was fine.
Chris K has brought this to us :
No, that would likely blow the walls out.
The Natural Philosopher was thinking very hard :
So it might be worth painting the exposed polystyrene with PVA as well as adding a barrier?
You were only supposed to blow the bloody doors off!
bet you can do the annoying nasal twang too?
Jim K
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