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  #1  
Old March 15th, 2012, 11:04 AM
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Wet vs Dry Sanding

Hey everyone,

Was just wondering if anyone have any advice about paint prep. I have done a lot of research preparing to repaint my 110. The consensus seems to be 400 grit or Scotch pad to roughen up the surface for the paint to stick. I picked up my PPG Concept paint from the local paint supply last week and they are telling me I should DRY sand with 400 grit using WHITE sandpaper, not to wet sand with the black sandpaper. WTF? Really? I have spent the last few months sanding my 110 and thought I was finally ready for paint. I know prep is everything when it comes to painting but isn't this a bit ridiculous? Wet vs dry, White vs black? It seems stupid but I am no professional so would like to ask for your experiences. Thanks in advance
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  #2  
Old March 15th, 2012, 12:08 PM
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My paint guy said to dry sand with either 320(?) or red scuff pads. I opted for the scuff pads for the hand work and paper for the DA.
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Old March 15th, 2012, 12:31 PM
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My preference is to wet sand with 220, acetone wash, then tack off, then prime with 2 part epoxy primer, dry sand with 400, tack off, acetone wash, top coat
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  #4  
Old March 15th, 2012, 12:44 PM
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Quote:
Originally Posted by o2batsea View Post
My preference is to wet sand with 220, acetone wash, then tack off, then prime with 2 part epoxy primer, dry sand with 400, tack off, acetone wash, top coat
OMG I have to resand after the primer? I thought you were suppose to paint after waiting for the flash time to get a chemical bond, no? I'm never going to finish this truck!
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  #5  
Old March 15th, 2012, 02:30 PM
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Hold on please

First, the difference between wet sanding and dry sanding given the same grit size is that wet sanding uses water as a lubricant between the sand paper and the paint surface. The original purpose of wet sanding is to produce a more uniform finish. Dry sanding will often cause skipping/catching and in the hands of anyone but talented artists/craftsmen is less uniform than wet sanding. It sounds like additional "work" but wet sanding actually makes your life easier and the result will be nicer. Some people can dry sand because they are artists or very good at dry sanding. But for the average person, wet sanding is recommended.

Sandpaper is just paper with grit adhered to it. The color of the grit is immaterial. What matters are:
1. Size of the grit
2. Waterproofness of adhesive (is it a wet/dry sandpaper)
3. Material the grit is made of.

A common material for sanding grit is aluminum oxide. There are other types of grit, such as silicon dioxide, silicon carbide, etc. For the purposes of painting, silicon dioxide and aluminum oxide are fine. Silicon carbide works well too, but more expensive and unnecessary for this application. That is like buying a $900 CPM-S90V full-custom titanium liner-lock folding knife to cut butter. OK, well, in this case a simple butterknife will do.

Generally speaking, do not bother buying dry-only sandpaper. Unless you are finding it for dirt cheap. The adhesive used on dry-only sandpaper will debond in wet-sanding and unless you have experience using lapping compounds, this is undesirable.

There are different types of paint jobs:
1. Two-stage primer/topcoat
2. Three-stage primer/color coat/clear coat
3. Powdercoat

OK, since you are using PPG Concept, I can assume that you are using a two-stage acrylic urethane topcoat over primer.

Regarding "chemical bond between the top coat and primer" - unless you are using some kind of extremely wicked chemistry, the different stages in a paint job should NOT have any interaction between them at all short of the intrinsic bond between the top coat and the primer as the top coat dries.

The primer should be completely cured before applying the top coat.

So, let's talking why you are using primer. There are several reasons, but the purest answer is this: You are attempting to create a uniform surface on which to paint. Forget about rust conversion and other gimmicky stuff like that. The most important part of the primer is uniformity. If you primer a flat metal panel, then the ideal condition is that the primer is totally flat, like the surface of a lake. Does this mean that it has to be mirror-polished? No. We will talk about that next. But it has to be uniform. That is what you are trying to achieve with the primer.

Let's talk next about primer surface smoothness. Surface smoothness is not flatness. Think about a piece of 50 grit sandpaper. If you lay it flat against a table, then it is flat, but not smooth. The surface smoothness of the primer has two qualities:
1. Up to a certain point, it will directly affect how good the surface smoothness of the top coat will be.
2. It will directly affect the strength of adhesion between the top coat and the primer.

Addressing point 1:
The top coat, especially on an acrylic urethane has the ability to hide imperfections - to a point. This is the leveling ability of the paint. Typically this could be as low as 400 grit as high as 2000 grit. Viscosity and surface tension have important parts to play in this. What I mean by this is that your primer could have a surface with 400 grit imperfections, and the top coat could then fill all the gaps on the surface of the primer to make a perfectly smooth surface. If your primer has a surface with 200 grit imperfections, then the top coat might not fill those 200 grit gaps on the surface of the primer and the primer's lack of smoothness could show through the top coat.

Addressing point 2:
The adhesion of the top coat and the primer has to do with surface area. Take a blanket and lay it flat on the floor. It will cover a certain surface area. Now take a larger blanket and make it fit in the same area as the original blanket. The larger blanket will be wrinkled. You will have more blanket covering the same area.

The adhesion between top coat and primer has a single value in psi/square inch. A mirror smooth surface is like the flat blanket. A rough surface is like the larger blanket. If you measure the microscopic surface area created by the microscopic fissures and grooves in the rough surface, then you will find that it is actually a larger total surface area than a mirror smooth surface.

Since adhesion = lbs/square inch, if the number of square inches goes up, then the adhesion goes up. This is why some folks sand their primer prior to top coating.

This is all regarding primer only. There are completely different rules for the top coat. For top coat, multiple layers have to be sprayed within a certain window of time and they indeed do chemically bond. Sanding must always be wet. Your final surface finish will have to be achieved with a dual-action polisher and cutting/polishing compound.
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Old March 15th, 2012, 10:37 PM
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Wow, thanks Ed!

So if I am reading this correctly, I should wet sand, prime, wait for it to completely dry, dry sand, then paint. Right?

I was told PPG Concept was a single stage paint. I am not doing a clear coat.
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  #7  
Old March 16th, 2012, 08:41 AM
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Read the spec sheet for your primer and paint. it will tell you exactly what you need to do and how long you need to wait.

For example the spec sheet for one of the primers they recommend with the paint you are planning to use says you can top coat without sanding

"Note: NCP270/271 must be sanded prior to topcoat application if allowed to
dry more than 8 hours."

I would still sand it as you will remove any problem areas (Dust / little objects or orange peel) and primer is super easy to sand. As a new painter you will end up with a better paint job in the end if you spend a few hours sanding.

Main page for PPG concept will have the list of products they recommend
http://www.ppg.com/coatings/refinish...px#singlestage

Tech sheet for your paint
https://buyat.ppg.com/refinishProduc...d-5ab5f079dc33
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  #8  
Old March 16th, 2012, 09:57 AM
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np, sorry for being so long winded

I guess to be brief, in order:
Dry sand old paint, clean, apply primer, let dry, wet sand to uniform surface, clean, topcoat, wet sand, polish
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Old March 18th, 2012, 09:26 PM
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Probably the best summary of DA polishers I've seen so far.

His other videos have some issues. He uses WAY too little polishing compound and never really does a good job removing scratches.
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  #10  
Old March 19th, 2012, 08:19 PM
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When you are sanding be very careful on how you position your hand to the direction you are sanding. The old school term is a Finger F$#%. Do not let your fingers dig a groove into the primer and or finish. That is why Ed went over the wet sanding deal, a lot less mistakes.
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