Sanding a hard body shell

[Kol__ 3377]

Hi guys,

I'm repairing a bashed up Grasshopper II body shell. What are these white hard shells made of? ABS plastic?

I've sanded down the scratches with 80grit paper and then 120grit. I don't think any filler will be required. Can anyone recommend a suitable finishing grade sand paper to use prior to painting with Tamiya primer? I'd like to ensure no scratch marks show through whilst still leaving a really good key for the paint. Maybe a 400grit?

Cheers

[Goudar 526]

ABS plastic.

I tried with #400 and #600 on my Blackfoot. It depend on final result you would like to achive, you can prime then use a #1000 wet sandpaper on primer then do the first coat.

Here the result (#1000 sandpaper) on my RX Memorial :

 

I'm pretty happy with the result, great glossy finish without any clear cloat.

[Kol__ 3377]

Nice one @Goudar, thanks mate

[Webphut 11]

8 hours ago, Kol__ said:

Hi guys,

I'm repairing a bashed up Grasshopper II body shell. What are these white hard shells made of? ABS plastic?

I've sanded down the scratches with 80grit paper and then 120grit. I don't think any filler will be required. Can anyone recommend a suitable finishing grade sand paper to use prior to painting with Tamiya primer? I'd like to ensure no scratch marks show through whilst still leaving a really good key for the paint. Maybe a 400grit?

Cheers

I would work your way all the way out to a Scotch Brite pad. The brown color pad. The paints and primers tend to work by a chemical reaction when being applied to plastics. With a Scotch Brite finish this will be more than enough surface disruption for adhesion.

400 grit? That might be too course, but you can try shooting the primer really wet. Make sure you see the primer flow before it flashes. If you don't get flow, might see the grains from sanding. If  you shoot it wet enough, but not so wet that it runs, It might cover the sanding marks. Try a 75% overlap and if it runs, try 50% and so on until no runs. 75% should get good flow and hopefully no runs.

The Lingo:

Overlap: the amount you shoot over the previous stroke. If you shoot a 4 inch strip, 75% would be 3 inches over the  previously shot area and 1" of non painted area. This might sound like a lot, but if you have a steady hand, it works out really good. 

Flow; If you watch the primer or paint you are shooting, you can literally witness the overlaps of primer and or paint merge together as they are being shot. 

Flash is when the chemicals in the paint and or primer evaporate after it has been shot and hopefully it is done flowing. You will literally see the wet look disappear. It usually starts from the outside edges and evaporates in a direction towards the middle. It wont be dry, but that is the effect.

If you get any runs, you can use a razor blade to slice the run off or scrape the run off after it is dry. There are You tube videos on how to do this. Time consuming but totally not detectable if done right.

 

Good luck and if it is done already, post some photos mate.

 

[Willy iine 18805]

@Kol__ I use Tamiya's sanding sponges on the styrene based hard shells.  Most hobby stores have them, even Amazon.

I usually start with the 400# and then by the time things are nice and smooth, the sponge is well worn to about 800#.  So I let off on the pressure to achieve a close to 1000# finish.   Billy's helmet I did this morning is exactly that.

I use TS paint which does not require any primer.  I only use the white primer when I need to get the plastic to a color closer to whatever the color I am trying to paint.  Then between coats I use either a worn 1000# or 1500# sponge to rid any blemishes including any dust marks, if any.  

BTW, when I need to remove a lot of plastic (like removing injection lines, etc) I first start off using a metal file of various shapes with mild pressure.  Then proceed with the 400# sponge.   Hope some of this sparks some ideas for your techniques.

[Webphut 11]

 

this was supposed to be up in my previous post, but anyways, this is the pads I would use.

[Kol__ 3377]

Thanks guys, really appreciate the advice. Nice one on the details and lingo there @Webphut and nice to hear some of the secrets that go into Billy heads @Willy iine. Looks like I've got some materials to source!

[Kol__ 3377]

Just a note to say thanks again guys. I picked up some of the Tamiya sanding pads and over the top of my previous 80 and 120 grit to remove all the scratches on the shell, I then did 400-600-1000 before laying down the primer. Seems to have gone pretty well so far. I think I'm a long way off from understanding and properly applying the techniques of overlap, flow and flash @Webphut, but learnt a lot from this one so far and onto the top coat now

Primered...

[Willy iine 18805]

@Kol__ Looks good.  I keep a small 10mm width slit of #1500 sponge (or a worn #1000) near me while I spray.  In between coats if I find any blemish I lightly rough it up and respray.  I remove any foreign matter like dust and cloth hair, but if it's just TS paint dust I don't remove the dust and continue on spraying as the following layer dissolves the paint dust, fuses into the new layer, and blends that roughed up area well (if not better).  

It's up to you how you spray, just thought I'd mention how the sanding sponge is very useful to me while spraying too. 

[Kol__ 3377]

Nice one @Willy iine, invaluable advice as always mate, cheers. I did notice dust from each coat when spraying, more so with the primer, so wiped it off with a 'lint free' micro fibre cloth between coats. I did try a quick sand after the first primer coat had dried, but it quickly and easily stripped off primer from a couple of the edges, even with a very light pressure. So I abandoned that approach and just went with the cloth wipe between coats. I hadn't thought of the idea that the dust would melt between coats, and I like the idea of just paying attention to specific blemishes between coats and addressing only those areas, cheers

 

[Willy iine 18805]

@Kol__ No problem, whatever works for you is best.  I probably layer each coat heavier than you thus do not burn through the layer with the sanding sponge.