Compounding - i'm not getting the results i want.

[julianf]

Im clearly doing this wrong, as, whatever i try, i cant get it truely scratch free when dark colours are tilted to the light. Always either scallops from my random orbit, or larger swirl marks, or even random deeper lines.

So, im spraying, then a day or two later, taking out any surface imperfections with 1000grit on a mirka DA sander.

I was using the hook and loop paper (800g) but i thought the scallops were caused by the hooks poking through the paper (?) so changed to 1000g expensive foam backed pads (mirka again, i forget the name - red and black things)

Today, i couldnt find anything other than 1000g and 4000g. 1000g seems to flatten off the job, and ill go over it with 4000g, and still, the little, i guess, about 8mm dia crescent shaped scallop scratches. Its like theres grid on the pad, but there is not.

I mean, grit is an exageration, as the scratches are so shallow, but they stand out.


Ive got compounding paste - G3, and a yellow foam pad. Indeed, it may even be this thats causing me the bother, i dont know. Im unsure if im supposed to hold the sander lightly, so the pad skims over the surface with high rpm, or press it hard down, so theres more vibration, and no rpm.

And then if i wipe the compound off with tissue, or microfibre, even if ive got shot of most of the scallop scratches, ive still got radial fine marks all over the thing.


Maybe these would be filled with some sort of wax, but i cant add anything like that if im going to print the job - it has to be good before hand.

I just feel that my technique must be all wrong, and am hoping someone can advise? Really what im wanting is a flat finish if i tilt to artificial light, and im a way off that.

Dont get me wrong, its not a dead loss. Ive attatched some photos below. But, again, in artificial light at the right tilt, these wont look perfect at all, and thats what i want.

Thank you.

ps. im not so bothered by the slight undulations in the surface (as shown by the straight lines of the solar panels in the shot below) but its the fine swirl marks that i want rid of.

 

[keithski122]

I always find when I flat and polish it never seems as shiney as straight from the gun, flatter with less orange peel but not as glossy.

 

[bert]

I flatten with 2k paper but you can still catch some grit or an edge causing a scratch.

Then 4k on a da before machine polishing

 

[julianf]

After 1000 (I know you said 2, but 1 is what I have) the surface looks uniform, but misted. Then after 4000 it has a glossy sheen to it, but you can see the scallop marks then.

Should that be the case? I'm not sure if I'm revealing them from the 1k or creating them later. I don't know if every stage just gets rid of some and adds more!

Also, should the pads (the foam backed 1000g or 4000g) be used wet or dry? They're close on a four quid each. I've been using them dry today but theyre just going to get gummed up, I'm sure.

I'm aware this is basic stuff. As I say, I'm sure I'm missing somthing so I'm wanting someone to tell me what I'm doing wrong! : )

 

[julianf]

...also

I don't have a non-random orbit polisher. Should I have one?

 

[bert]

I found 1500 leaves massive scratches and takes ages to polish away so 1k is going to be worse.

It all needs to be done wet of you are dragging all the stuff your sanding off across the paint causing more scratches.

I have both types of polisher but end up using my rotary more as its quicker than the random orbital

 

[norsa]

By 1000g im assuming 1000 grade grit? If so i would say its a bit agressive unless you have a very peely finish. Better to start with 1500 but crucially, you need to take the scratches out by doubling the grit in the same way as you do with filler. i.e in my case 1500 3000 6000 then compound. You will never take 1000 scratches out with 4000. Hope it helps.
Nick

 

[julianf]

Thank you.

Can i ask what abrasives i should be using?

Ive just looked up the name of what i have - Mirka Abralon

My DA long ago lost its hooks (was 2nd hand, so not really by my use either), so i have velcro (tm) hooks. I dont really think they poke through the paper, else, im sure, they would be long gone, but that was why i changed to the Abralon foam pad things.

But, again, theyre almost £4 each, so im not sure theyre sensible, so id welcome recommendation?


When using water, should i be just misting on the job, or proper wet to wash stuff away?

Thanks to all - i feel like i might actually learn somthing about this!

 

[bert]

I use a block with wet and dry and hand flatten the paint with 2k grit. The 4k is abralon or trizact discs.

The wet and dry I dip it in a bucket, the abalone I use a water spray bottle

 

[ajlelectronics]

The trick is go through the grades and not go large jumps. It depends on the original finish as to where to start. If it is peely, then 1000 grit might be about right, otherwise I would start at 1500. It is imperative that everything is perfectly clean and that the water is changed often. I might go up in 500 grit steps, 4000 will just skim over the top of 1000 grit scratches. G3 can be a bit fierce too, I use a 3M equivalent, followed by a very fine compound. Lots of water changes and don't try to use the same pad for compounding. Hard pads, aggressive cut, soft pads, gentle cut.

 

[steveo3002]

too big a gap between 1000 and 4000

 

[MetalMonkey]

It would take you a long while to polish up 1000....
1200 is normally course enough to take out peel if it isn't that heavy. I tend to use 1200, 1500 and 2000, and optionally a 3000 trizact disc.
I can polish from 2000 or even 1500, but it is quicker with the trizact disc. I wouldn't imagine there is much point going finer than 3000 as it doesn't take much buffing to bring that up.

...also

I don't have a non-random orbit polisher. Should I have one?

If I read that right, yes you want a rotary buffer.
The DA ones are fine for detailing and small corrections but you need to get some speed and heat into the paint to polish it up quickly. Just a cheap silverline rotary is fine - its all I use.

You will need to practice with it if you haven't used one before, as they can be very aggressive and quickly burn through paint, especially on edges.
However, it should leave you with an excellent finish once you have mastered it.

G3 is fine, but I think there are better - I use Menzerna. You will want a couple of pads and grades of compound, a harder one for initial compounding and a softer one for polishing

 

[Ruffian]

Can get 1500/2000 grit in da discs (got some 1500 here if wanted to try a couple) and 2000 hand sand.

Only other thing would be soft backed 3000 trizact discs (still go on a da but for ultra fine polishing.

What polish are you using though?
As think that could be part of the problem.

 

[julianf]

I found some 2000g paper, so tried this -

1000g
2000g
4000g

and then this, on a yellow sponge (i tried a mop thing also, but that seemed worse) -

This is the result that im getting -

I had to put the tissue on there to get my camera to focus, but you can see from the edge of the tissue, where its frayed and you can see the tissue fibres, that what im talking about on the paint is small. What i mean is, if you use the tissue fibres to give it all some scale...


But i just cant seem to get rid of them.


The mop thing (some Chinese lamb maybe) gives the same sort of result but maybe a bit worse, which does make me wonder if its the final stage that is the problem, and thats where i should be changing things? But maybe these marks are just carring over from an earlier stage.


Ive just realised that what i should do is run the sponge / g3 on some paint that has not been sanded, and see what the result is then. If im left with a smooth finish, then the marks are from the sanding, but, if im left with somthing like the above, then that would be conclusive, i guess?

 

[steveo3002]

was it 100% clean when you sanded it ...kinda looks like some dirt was on the pad

i prefer wet sanding , you can hear when its got dirt on and also be squeezing a sponge so theres a constant flow of water over the work

 

[ajlelectronics]

That is deep and coarse. I think you might need to go down to about 800 to get rid of that and start again.

 

[julianf]

That is deep and coarse. I think you might need to go down to about 800 to get rid of that and start again.

I appreciate evreyones advice, but my point is that if i do the same thing again, the same result will happen. I know this with certainty, as ive tried variations of this many times with the same result each time.

There is zero point in doing the same thing again without changing something. I'm asking specifically what i should be changing.

The prior suggestion was to double the grades each time, which is what ive just tried, now that ive found some 2000, and ive ended up with the same result again.

So, what should i change next time?

 

[julianf]

was it 100% clean when you sanded it ...kinda looks like some dirt was on the pad

i prefer wet sanding , you can hear when its got dirt on and also be squeezing a sponge so theres a constant flow of water over the work

Ive wondered before about stuff caught in the sponge, and so i swapped the sponge for a new one, and still the same.

 

[mtt.tr]

My only in put would be to make sure you keep the paper lubricated otherwise the Material that's removed scratches what's left.

 

[MetalMonkey]

They look like the sand scratches which haven't been removed by the subsequent grades of paper.
Looks like they have been put in by the first grade of paper, and not removed when you changed grade - each grade of paper must remove the scratches from the grade before.
2000 is never going to remove 800 grit scratches (well not in any reasonable time frame).

What you could try is blocking a small area by hand with wet and dry (kept wet) and working in a cross hatch motion.
You will have a lot more control and be able to see when the swirl scratches have been removed, then work through the grits on wet and dry and see if that polishes it up.

Another thing to check is the type of sanding discs you are using - good quality discs designed for automotive finishing will have very consistent grits.
Cheaper general purpose discs might have large bits of abrasive mixed in which impart a lot of deep scratches.