Sanding techniques and help needed

[tubachris85x]

So I've found it quite hard to understand why, despite using primer, sand, repeat that I'm getting some really bad surface results after I go ahead to paint, despite diligantly checking the surface.

For the most part, I have to use my mouse sander with 120 grit, to strip down the bad layer of paint. After it's stripped, I hit it with 220 grit (by hand) for a good amount of time. After that, I put down some primer, then sand with 220 grit, and then wet sand it. After all this, despite how smooth the entire surface physically feels, I go and put the paint down, and out of nowwhere, the sanding marks from when I took the mouse sander to it, starts appearing through the paint, leaving a rough and jagged surface on the paint, therefore, causing all the prep work to have been for nothing.

Why is this happening? I feel as if the primer isn't doing its job or I'm completly missing something here

 

[robstyle]

whats the material your sanding, is it a level surface? Not all primers are filler primers. A filler primer will do just that, fill small swirles and lines. Regular primers tend to go on thin, a filler primer will go on thicker.

Since your using a power tool for the sanding, it could be uneven pressure on the surface and you dont even realize it. And, although 220 is a good standard leeway in grits, go for a still finer grit and possibly even another finer grit after that. The end result once painted is only as good as the prep.

 

[tubachris85x]

whats the material your sanding, is it a level surface? Not all primers are filler primers. A filler primer will do just that, fill small swirles and lines. Regular primers tend to go on thin, a filler primer will go on thicker.

Since your using a power tool for the sanding, it could be uneven pressure on the surface and you dont even realize it. And, although 220 is a good standard leeway in grits, go for a still finer grit and possibly even another finer grit after that. The end result once painted is only as good as the prep.

All of the material is plastic, one is ABS, the other is a semi-flexible plastic, used for sports protection. I've been using the rustoleum automotive primer. Does home depot/Lowe's sell filler primer? I've yet to see anything else

I've thought about getting finer grit just to be secure

 

[robstyle]

If your sanding plastic/abs, do it by hand and you will need finer grits to work out the swirls and gouges. Im not sure if the places you mentioned have filler primer but you could also contact a larger hobby shop (not michaels or joannes) that sells scale race cars and the like and ask for it.

 

[noble]

After spray primer,

Wet sand with 320, then spray primer again.

Then wet sand with 400.

I don't recommend sanding beyond 400 if you plan on painting it.

(paint won't stick to a super smooth surface)

 

[stapleton13]

Something else not a lot of guys do is use guide coat to make it easier to see the surface imperfections.

After you prime with Grey fill primer, and it dries. Spray a mist of guide coat onto the part. You can get it at the auto paint store, but even something like black primer will work. After it dries you can sand, and right away you will begin to see all the surface imperfections. The goal is to sand the primer until you don't see any of the black guide coat.

I'll see if I have some pictures at home. If not I take a few shots tonight. I have about 5 things ready for sanding.

Myron

 

[tubachris85x]

If your sanding plastic/abs, do it by hand and you will need finer grits to work out the swirls and gouges. Im not sure if the places you mentioned have filler primer but you could also contact a larger hobby shop (not michaels or joannes) that sells scale race cars and the like and ask for it.

I'll contact the local hobbytown to see about it, and will check out the hardware stores

 

[defyitall]

The only thing I can recomend is not using a Mouse sander on plastic. The sanding marks it leaves can be quite deep and in a regular pattern. If you need to use a sander a random orbit would be better. Plastic stuff is better done by hand.

 

[E-Wan]

I buy my filler primer at Pep Boys, so you could try auto places too.

 

[defyitall]

Just don't mix Duplicolor auto paints with Rustoleum or Krylon. I've had wrinkling problems with that combo.

 

[MsngPcs]

After spray primer,

Wet sand with 320, then spray primer again.

Then wet sand with 400.

I don't recommend sanding beyond 400 if you plan on painting it.

(paint won't stick to a super smooth surface)

That! You need to final sand with a higher(smoother) grit. :thumbsup

 

[Jarvis B]

One trick I use is if the part your painting is low detail sand as smooth as possible going through the different grades of sand paper , when it seems to be as smooth as you can get it use a Gloss Black Duplicolor paint from Pepboys , several coats and let dry for a few days to let it cure fully. Then prime ,wet sand and paint the color you want .

 

[tubachris85x]

Thanks guys for all of your suggestions! We've been trying alot of the techniques mentioned. We are going to try some more of the methods you guys mentioned, but part of the issue is we feel it's just the plastic not allowing the paint to adhere. We are even considering getting the pieces airbrushed, rather than using regular spray cans.

Either way, we are both pretty frustated with it..

 

[Rebelscum]

I thought I'd chime in here as I do a lot of sanding and finish work in my shop.

If the original problem is that after painting marks show up from the mouse sander, then the issue lies that sanding with higher grits was done too soon. Filler isn't really the answer to the problem and can introduce its own problems since it can add quite a bit of thickness to the piece, and could even cause you to lose detail.

The goal of every stage of sanding is to sand the entire piece with the same grit until the scratches that grit leaves is absolutely, completely even on the surface. Even one tiny line that's deeper than the rest will not go away with subsequent sanding. At best the edges of it will be rounded over, but it will be still be there and will show up later.

I always use a light held at an acute angle across the piece to check the entire surface for imperfections.

Once you are sure you have even scratches on the entire piece, blow off or otherwise remove all of the dust from that procedure so you don't carry that grit with you and move to the next finer grit of sandpaper, which can vary by application, but I generally use 80, 120, 240, stopping there if it's wood that's being stained, going on further if it's being clear finished, or if painted, I'll go to 320 then prime.

If at any point you see a scratch from a previous grit appearing as you are sanding with a finer grit, you have to go back and do that previous grit again.

At the priming level I use 320 for a couple coats and finish at 400.

With this process I regularly finish MDF to mirror gloss black, even on the edges.

 

[noble]

You might be using a bad primer.


Real auto body supply stores (not pep boys) have special etching primers
to adhere to bumpers(plastics/metals,etc.)

Also, never sand with only your hands! Pro stores sell awesome sanding pads.

I don't recommend using electric tools for fine sanding.


stores like "finishmaster" are ones to look for.

 

[stapleton13]

Here are the pictures I said I'd show you. Sorry about the delay.

All the above advise is right on target. I also heard mention that the problem is the paint may not be sticking. Many auto paint stores also have sprays that are adhesion coats.

I always use guide coat to see the scratches, and imperfections. I even use it when sanding filler. It really helps find those low spots, and never use your fingers as a sanding pad. I buy foam sanding blocks at the paint store, and cut them down to the sizes I need.

Myron

Here is what guide coat looks like when sprayed over primer.

Here is an example of how it shows imperfections.

 

[Tetmatek]

guide coat is the way to go! Another way to do it is to either buy 3m dry guide coat or grind some charcoal into dust and rub it on the primed surface. The dry stuff works way better than paint since it goes on very even and soes not gum up the sand paper. I will post a picture of what it looks like when I get to work monday. Being a powder it will go into little nooks and crannies of a part.
Most all paint companies have started recommending 500P grit for finishes to lessen sand scratch swelling.

 

[stapleton13]

guide coat is the way to go! Another way to do it is to either buy 3m dry guide coat or grind some charcoal into dust and rub it on the primed surface. The dry stuff works way better than paint since it goes on very even and soes not gum up the sand paper. I will post a picture of what it looks like when I get to work monday. Being a powder it will go into little nooks and crannies of a part.
Most all paint companies have started recommending 500P grit for finishes to lessen sand scratch swelling.

I have used the powder type as well. They are both pretty cool products. A painter friend I have told me one time. Guide coat is your Friend.