Sanding, Primering Car before Paint??

[juantonwan]

I roll painted my car about two years ago. I want to get it repainted but don't want to pay a whole lot.

I was considering Maaco, but I want to sand and possibly primer it before hand. I was curious what I should do.

Should I sand the entire car down to the original color. Then leave it all scuffed and let Maaco paint it?

Or should I sand it down to original color, then primer it, then let Maaco paint it? Thanks

[Ivan141]

Only primer it if you're going for a lighter color (if you're afraid it will shine through). If you primer it, dont forget you need to sand that again.

[juantonwan]

Ivan141 Wrote:Only primer it if you're going for a lighter color (if you're afraid it will shine through). If you primer it, dont forget you need to sand that again.

So would it be best just to sand it down close to the original color, then just take it to Maaco roughed up?

[Ivan141]

In theory that should be enough for the new paint to stick.
The biggest benefit of fresh primer before paint (not counting bare metal preparation) is that you get a more uniform paint coating with less layers of paint. But this mostly holds true for lighter colours.
There is nothing wrong with primering it, but if you plan to do it, use good primer.

[juantonwan]

Ivan141 Wrote:In theory that should be enough for the new paint to stick.
The biggest benefit of fresh primer before paint (not counting bare metal preparation) is that you get a more uniform paint coating with less layers of paint. But this mostly holds true for lighter colours.
There is nothing wrong with primering it, but if you plan to do it, use good primer.

Thanks for all the info!

1) Should I wet sand or dry sand? What type of sandpaper should I use?

2) I am not going to primer, so do I just sand as smooth as I can? - Do you use a rough sand paper first, then smooth as best I can with a less rough sand paper?

3) Should I make sure to sand off the Rustoleum enamel entirely?

[vlgi]

The difference between wet and dry are not that great, wet sanding usually causes less dust, which I guess is beneficial if you're painting don't want that dust settling on the paint when its drying. Wet sanding also reduces heat generated when sanding, so in some applications that is useful.

When sanding if there is a lot of sanding back to do then you would start with a rougher (not rough really) and then a finer paper. I wouldn't expect you'd have to go too rough, a fine paper is fine.. Uhm I forget grades of sandpaper, was it around 4000 somethings, hrm that seems a bit high, I'm helpful yeah. I think something like P800 - P1000 is your rough sandpaper, and P2000 - P2500 is your fine sandpaper.

Also when getting sandpaper get the kind that is treated to avoid getting clogged with dust, which can really help lengthen the life of the sandpaper. Don't think it works with wet paper so much tho.

Don't know about the rustoleum stuff so much I guess its a treatment to protect against rust as well so its meant to stay there, just get a key and a smooth surface to apply paint.

Before rubbing down the car its probably worth cleaning the car fully, then using a clay bar to strip off any residual junk to get directly to the paint surface, and then once you've done that use a fine grade P2000 etc. to get a key for the paint. If you find P2000 is too fine you can move to more coarse papers, I guess in steps of 500 maybe.

[roman]

You should definitely primer your car before painting!

Primer is basically an intermediate layer that's designed to stick to what ever is underneath, and give a good surface for the new paint to stick to.

Your paint will probably fall off, painting directly over existing paint!

The idea of wet sanding is that having a constant flow of water over the surface removes all of the dust particles etc that would normally be in between the paper and the surface...

If you are using a really fine grade of sand paper, the particles in between can be courser than the paper itself, and leave scratches.

So IMO it's only really important to wet sand when you are giving paint a cut back before polishing it up.

[Ivan141]

Havent had any problems with paint falling off myself, but I guess it really depends on the paintsystem you use. If you use something that's not matched to the old paint you will get problems, but that will also hold true for a ground layer of an incorrect paint type. You need a sealer when switching to a different paintsystem.

[eight-six]

Quote:Primer is basically an intermediate layer that's designed to stick to what ever is underneath, and give a good surface for the new paint to stick to.

Your paint will probably fall off, painting directly over existing paint!

Not true! By keying the existing paint layer it essentially acts like your primer coat; the new paint bonds to that. As long as the existing paint layer is good then go for it.

No different to if you spray primer on top of old paint then you still need to ensure that the surface underneath is keyed to accept the primer – other wise that wouldn’t stick either.

By keying the existing paint you can save a lot of time and 1 paint stage.

Quote:1) Should I wet sand or dry sand? What type of sandpaper should I use?

Wet sand paint work – the water helps the paper slip over the surface unlike dry sanding and helps prevent clogging. Use a sanding block on the final coat colour sand and some fine grade paper – 1200 wet and dry then you want to polish with a DA with something like g3 and then g10 to get it looking nice and shiney. To get a good finish out of the gun on the last coat increase the amount of thinners and give a light coat. The thinners should cause the paint underneath to flow out and flatten down which will give you less time colour sanding later on

You shouldn’t really wet sand primer – as it absorbs water which can then cause issues when you lay the paint down over the top. If you primer it properly and lay down a nice flat coat then you should be able to simply rub the primer down with a scotch pad to key it ready for paint. If the primer hasn’t gone on very flat then you will want to block sand the primer to flat it down and key it prior to paint.

Quote:2) I am not going to primer, so do I just sand as smooth as I can? - Do you use a rough sand paper first, then smooth as best I can with a less rough sand paper?

No you don’t want to burst through the paint layer – you just need to key the surface. Clean the car first, dry it and then use a scotch pad to key the old paint, as long as its good paint then this should be fine (as long as the new paint doesn’t react with the old paint). Remember you just want to dull the surface of the paint, not rub through to bare metal – pay attention to edges, its easy to break through the paint there but also easy to miss and not properly key the surface which will give you problems later on. If you don’t use a scotch pad than use 800 grit W&D and water – obviously dry, tack cloth and panel wipe before paint

Quote:3) Should I make sure to sand off the Rustoleum enamel entirely?

If its good paint just key the surface and paint over – key thing is what you intend to cover it with? Remember though not all paints are equal – depends what you intend to paint with and what’s already on the car as some paints react with others. I know – I have just sprayed up some of my N2 kit and the panels that had spot repairs with 1k acrylic enamel has reacted with the cellulose I sprayed them with.

Not a worry if you have incompatible paints you can get a product called barcoat which is basically a sealing layer over the old paint, meaning you can put your new paint on top and not get a reaction. Unsure if this is 100% fail safe will find out this weekend when I sort out my N2 wings.

From what I understand Rustoleum is pretty inert with everything (being applied over celly / 1k/2k etc ) so would assume it works the other way around as well. Would you be spraying it again with Rustoleum? A professional restorer advised me to try it instead of cellulose – only issue was I couldn’t get it in the colour match I wanted but they do A LOT of colours, maybe worth a look and zero chance it will react.


Before you pray use a tack rag and panel wipe to clean any crap / residue off the paint – be fussy!


FYI I am no paint pro, this is just what I have been advised from someone that is and I thought I would pass it on – I am painting my car for the first time and ignoring the unfortunate paint reactions I am getting some awesome results!