Advise on sanding tamiya putty

[kcirick]

Hello,

I'm building a kit and to modify a little bit the body, I used tamiya putty. Putting on the putty seemed to have gone okay, but when sanding down, I cannot seem to get a smooth surface. I always get a bumpy imperfect finish after I sand it down (See the attached image).

I am using a 320 grit paper to sand it down after letting the putty cure for about an hour. I tried to repeat it a couple of times but without much success.

Is there something I should be doing different?

Thanks in advance for any advise!

EDIT: Attached picture isn't showing... See the picture below:

[hirofkd]

Looks like you are using Tamiya white putty, which is only good for thin application, like filling scratches and gaps. Also it doesn't dry in 1 hour. In fact, it may never dry if you applied it too thick.
What you need is either epoxy putty or polyester putty. Tamiya makes both, but you can find alternatives easily. Epoxy putty may be marketed as plumber's putty, and automotive body filler is essentially the same as polyester putty.
These kinds of putty cures by chemical reaction, so it's good for thick application. Epoxy cures in about 6 hours, while polyester putty takes only 10 minutes. Another good thing about these kinds of putty is that the shrinkage is negligible, while basic putty shrinks as it loses the solvent in it.

I'd use automotive body filler for this type of modification. It's cheap, available at any auto parts store, and it cures very quickly, and is ready for sanding in less than 10 minutes. The tricky part is that both epoxy and polyester don't adhere to plastic very well, and you'll probably have to remove the cured putty and re-attach it with super glue.

If you have to use Tamiya putty, don't apply any thicker than, 0.5 mm, then wait for overnight, then add another layer. Keep building layer by layer, and eventually you'll have enough thickness, but I really recommend epoxy or polyester putty for any type of body modification, and leave basic putty for finishing work.

[kcirick]

Thanks for the advise. Good to know the differences between the kinds of epoxy. I should have researched it a bit more before buying the first putty I saw in a store...

However, now that I have applied most of the putty I was planning on applying, it seems counter-productive to remove what I have applied (plus, I've read that removing the applied putty is quite hard). Is it necessary that I remove the exiting one? You mentioned that if applied too thick (which I did), it will never cure. Would this become a problem when I paint it (primer first, of course)?

In case I want to keep using the white putty (which I am inclined to do at this point), I'm coming back to the original question: How do I sand it so that I get a smooth surface? Any special care I should follow?

[nugundam93]

yes you do have to remove them all to ensure that you get all the putty that might not cure properly. it you don't, it will become a problem later as you'll have the hard outside putty and a softer inner putty which could crack (or cause the outside to sink) and cause a lot of problems.

as for sanding it smooth...wetsand it first with the 320 until all you see are recessed areas (your pic shows a lot of raised areas). fill the recessed areas with putty, let dry, then sand again. eventually you'll have to use finer grits to smoothen the putty surface. i do prefer wetsanding to control the dust that would've flown around if i didn't add water, and to keep the putty from fouling up early the sandpaper.