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TURK
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Hi all, I'm in need of a little help guys ........ I watched ( and learned a great deal ) from Paul's 'Airbox' video on YouTube, I purchased and currently using the Low Density 48Kgs/m3 PU foam blocks, to get my 'form'. I then put a skim of body filler to harden the 'form' that will eventually be my plug for casting a resin mould. After the initial body filler 'skim' had hardened, I applied a few more layers of body filler to build-up the form to the dimensions I wanted. So basically, my plug now has about 3mm of body filler on it that needs to be sanded down smooth. Unfortunately, that isn't what happened ........... the body filler ( known as 'Bonda' ) turned out to be more like a rubbery texture !!!  as we all probably know, it's impossible to sand down bubble gum !
Any of you know why it did this ? .......... did I overdo the 'skimming' of body filler ?
The plugs have now been drying for four days, but the texture of the body filler is still quite soft, to the point where I can push my finger nail into it. This was the original 'skim' of body filler >>  Then I applied a little more on to build it up as it were, so I could sand down the contours >>  So, another question if I may ......... As I've never had any troubles with this sort of body filler, I use it all the time ( mostly on MDF plugs ) and never had any issues with it. D'you think there's been kind of a reaction with the PU foam ? should I have applied a few layers of Pattern-Coat Primer to the initial 'skim' of body filler ? or can I simply apply Pattern-Coat Primer directly onto the PU foam ? ( sorry, that's three questions ! )
There's loads more info in the following link, and the entire procedure here > PU foam plugs
Many thanks for your time. |
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FLD
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Could the activator have gone off or be old? I've seen this with filler before but only when it's really old. More activator seems to cure the problem.
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Warren (Staff)
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I would agree that something has gone wrong. Even the bumper fillers can usually still be sanded.
I would get it somewhere warm if you can. 30C plus for a good few hours might just be enough to push the cure the final bit. If not, you may be best using very coarse paper to grind it off then put on fresh.
You could try overcoating with Pattern Coat Primer, however you would need to take extra care you don't sand much into the first layer of primer as you might easily break through to the bad filler.
Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant
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TURK
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Thanks guys, efficient and rapid response time as usual  I used that particular filler and hardener only recently .......... worked fine on the MDF. Seems a little odd to me that the very first skim of filler cured OK, then a couple more 'skims' ( build-up ) of filler, and things really went awry ! I had to place the plug in front of a radiator for three days for it to dry, but now it's began to crack ! Sanding was impossible, I tried 80grit sand paper but it was sticking to the filler and kind of creating 'balls' of filler .......... it felt as though I was trying to sand chewing gum. There was no sanding dust at all, it was simply accumulating into the sand paper. What's the 'bumper filler' you speak of Warren ? Basically what I'm trying to do is, to get these plugs to a very accurate dimension. If you were making these plugs yourself, would you cut and shape ( sand ) the plugs as near to the dimensions required, then apply Pattern-Coat Primer directly onto the PU foam ? TURK
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Hanaldo
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I'd wager you simply didn't put enough hardener in. Happens to me from time to time, despite having done this professionally for almost 5 years.
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TURK
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Hanaldo (07/12/2016)
I'd wager you simply didn't put enough hardener in. Happens to me from time to time, despite having done this professionally for almost 5 years. |
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Warren (Staff)
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TURK (07/12/2016)
What's the 'bumper filler' you speak of Warren ? You can get slightly flexible fillers for car bumpers and other flexible panels. However, it is still sandable.
Warren PenalverEasy Composites / Carbon Mods - Technical Support Assistant
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Hanaldo
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Turk, there's no reason you can't coat with pattern coat straight away, however it isn't really any easier or more accurate than skimming with body filler. Keep in mind that when skimming with body filler, you aren't trying to build thickness yet. All you want to do is fill the porosity of the open cells of foam, as well as any gaps and cracks that may be present in the foam. Pretty much as you did in the photo of your 'original' skim of body filler. So you're on the right track. Then instead of applying more body filler to do the contours, apply the pattern coat. Flat down, fill any low spots that can't be sanded out, and apply another coat of pattern coat. Repeat until you have a thick enough coat of pattern coat for it to be completely flatted down. Then mould or proceed with gloss coat. It took me awhile to figure out that your foam form isn't really where you should be going for dimensional accuracy. Obviously it needs to be close, but you don't want it mm perfect yet as you will just continually sand through to it.
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Dravis
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Ideally your plug foam core should be about 1 -1,5 mm smaller in all dimensions, and then the pattern coat filler should add a few mm to allow sanding it back to the perfect dimensions.
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TURK
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Thanks chaps, makes a great deal of sense that 
I tried to fix my previous plugs, but I think I was flogging a dead horse !  So I started again.
The nice people at 'Easy Composites' kindly sent me some more PU foam .......... I'm in a tiny village in central France, but it's not a problem for them .......... I had to pay for it thought !
I'm now using the slightly denser PU foam, 96Kg/m3. You could say it has the consistency of Balsa wood, great to work with. The final shape I'm trying to achieve is quite intricate, in order to have a vacuum forming friendly shape I can't have any negative angles, straight vertical side walls are also an issue, but I really need that 'footing' it's part of the area that will be bonded to the headlight retainer >>
And as advised, I applied the Pattern-Coat Primer directly to the PU foam ......... it's worked very well indeed. When that cures I'll sandback and fill any gaps and low spots with body filler, then a final Pattern-Coat Primer application >>
I'll let you know what happens .........
Many thanks all for your help and advice guys. |
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