Hi folks,
I want to get a glass-like smooth finish on my large scale foam model- like a car panel or bonnet.
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I have followed the auto industry- I used "spak" filler , primer filler, blocked it out for straightest/ flattest possible curves.
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And sanded with varying grits from 60 right through to 400 and higher, and even tried sanding the paint. But as soon as I put "gloss" or "clear coat" over base coat, I still see all the little dents and uneven highlights.
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Is this the nature of foam (as opposed to car metal)? I need it perfect as I want to make a mold over it.
Your solutions?

What kind of foam are you covering?

You're trying to put a mirror finish on a part using 400 grit?

Are you hoping to keep the model itself in flyable condition, or are you only using it to pull a mold? The bulletproof solution is glass, fill, sand, fill, repeat, polish, prime, paint, but you'll be doubling the weight of any foam part in the process.

If you have to work with a bare foam surface, you gotta treat it like it's a woman, not sheet metal.

Probably the nature of foam since it will be compressing/decompressing with the sanding pressure, and stiffer with the edges of the cell structure which will telegraph through the paint.

Agree with Piece about adding a fiberglass layer. Even 3/4 oz would be a be improvement.

Thanks for reply Piece, CGordon, Ward.
400 Gritt prior the paint job- for keying. 1500 (to 3000 )grit post. And yes I've repeated fill, sand.
... So I guess glassing is the only solution.
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Since I'm pulling a mold off it (also f/glass) weight is not a restriction.
But if I did want to keep the model flying, what's the alternative?
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The other problem is bubbling of the paint, despite extended drying between spraying thin coats.
Cheers-

If you want to keep model flying make a cheap mould of it as is.
Use mould to build new plug.
Sand and polish it to perfect.
Make perfect mould

That would work too. But I would recommend slathering a thick layer of epoxy on the model. Then sand it down to desired shape starting with 400 and go down to 3000 grit. Then, when you put your wax and PVA mold release on the plug, the mold surface should be pretty good. You will still need to work the mold surface also to remove the PVA lines. If you really want a mirror smooth mold, you'll need to CNC mill the molds from Aluminum then hand sand down to 3000 grit. Also remember to paint the mold with epoxy before laying any fabric in. Hitting it with a blow torch real quick can also help pop bubbles. Or you could pre-treat your epoxy in a vacuum chamber if you have one. This also removes bubbles.

Hope this helps.

Sean

Thanks Sporter, Gaza.
I've taken the fiberglass route- a thin layer. I guess its the analogy of building your house on a rock or on a sand pit. Still have to be careful not to get bulges/ waves in the FG. It's taken several fillers/ repeats to block/ smooth out- looking good so far...
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Sporter..I meant bubbles in the paint.. (spray paint more so than roller).

High build undercoat is great to finish fair surfaces.