Hi Simon,

You could try a thin mix of “Polyfilla” (Or similar) painted on.

 

Les.

I coat lost foam casting patterns with a thin mix of the sort of plaster used for jointing drywall (a small sack goes a long way). The mix is about the thickness of cream, and the pattern is simply dipped in it and allowed to ‘drip-dry’. I wasn’t trying to get a good finish – it is to prevent the casting sand collapsing into the mould at the instant the hot metal replaced the foam. Lots of people recommend latex paint, but it never covers properly, as Simon noted.

It might be possible to do this, then clean up (sand?) the plaster, and paint afterwards – for casting, I never cared about the outside finish. I assume you are using the ‘hard’ dense coloured foam used for building, which can be machined and sanded very smooth with care, not the white packaging type foam.

 

Another consideration might be that this stuff has horrible ‘attractive’ properties due to static electricity (I think) – I find I can get it apparently extremely smooth to the touch, but looked at closely in a strong light, it is covered in the dust produced by sanding it – maybe you can take it into another room and blow the dust off.

Simon,

 

I also fly r/c models (I’ve also flown with and know Clive the prev poster)…

 

So what he said…..Brown paper covering with diluted pva…us modellers use it all the time…. A fellow flier we both know as Mac, swears by it and paints his with the small sample pots obtainable from the likes of B&Q….

Hi Simon

 

I second Bob’s recommendation. Emulsion is cheap, inert and thick enough to fill imperfections. It dries quickly and several coats can be applied. Leave to dry thoroughly and then sand to get a good finish. you can then paint with whatever finish tickles your fancy,

 

Regards

 

T

How about foam on steel rod for support then mount in lathe between centres with hot wire as cutter, turn slowly by hand.

 

 

Billy.

Hi Simon,

My understanding of what you are trying to produce is a piece of foam 10 mm dia and 100 mm long with 3 mm bamboo stick through the centre. If this is correct then what about inserting the bamboo into a larger diameter piece of foam then making a lathe like machine that rotates very slowly an having a hot wire cutter parallel with the axis. with the “lathe” rotating feed the toolpost holding the hot wire into the foam until it is 5 mm from the axis (Against a preset stop.) and and allowing it to rotate one revolution. A variation on this idea would be to make “lathe tool” from a “U” shaped strip of metal that was heated by the hot wire method. This would require you to modify a transformer to give a very low voltage at a high current. (Probably a one or two turn secondary of very thick wire.) The foam could then be turned like a normal lathe but a very low speed. Probably about 3 rpm.

One of the old fashioned instant soldering irons could provide the transformer. A suitable strip of metal could be the strips of metal that are in the groves of a windscreen wiper blade.

 

Les.

Hi Billy,

Sorry for not reading your reply. I must have skipped past it. I agree on the toroidal transformer suggestion particularly for very low voltage requirements.

 

Les.