Barry K / Martinsr :How long between wet sanding and painting? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 11-26-2005, 04:30 PM
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Barry K / Martinsr :How long between wet sanding and painting?

Whats the deal with all the varying times between sanding and topcoating primers? Some products say final sand with 400 or finer and no time given. Tec prime martin senour is sand with 600 and 7 days from sanding to paint (over 7 scuff with gray scuff pad). Tint prime martin senour is 3 days. Some give one time for dry sanding and another time (usually shorter) for wet. Just curious what everyone else is doing. By the time I wetsand a body, clean the shop, wait for the car to dry and mask it my day is gone. I usually come out the next day, go over car with gray scuff pad, blow off, wipe down with wax and grease remover, blow off, tack rag, blow off and paint. Is the scuff pad necessary or not within a day or two. I seem to remember the martin senour times were much shorter a while back than what they have listed now. What exactly changes within the primer over time? Is it just to cover the manufactures against possible surface contaminants that may get embedded in the surface over time. I never quite understood the reason for the different times.
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Old 11-26-2005, 09:24 PM
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What you are doing is overkill.... There is no reason to sand a car today, get the shop in order and re sand tomorrow before you paint.... The next day all you need to do is tack the car off before you spray the base...

Letting primer set helps let it cure all the way,, most in the industry take like 2-3 days to cure, unlike the SPI turbo primer that is cured out totally in a few hours

Every manufacture has different top coat times, sand times etc... all depends on the chemical make up and the ISO's in the hardener etc.. Barry correct me if I go astray!!!

I would say the reason you have a shorter time wet vs dry is paper loading... you can sand gummy epoxy in a couple hours wet... but not very well dry..You always have to remember the top layer of primer/clear is going to be the driest and hardest... everything underneath is softer and full of solvents waiting to escape.. When you cut the top layer off you are allowing for a much faster cure. It is great you sand one day and paint the next!!!!

As far as I understand it, base has a physical adhesion to the primer...

Sand paper grit choice is like buttholes... everyone has one... you will see some people and company's recommend what I and many others feel to high of a grit, and the flip side with my views..

There is not a reason I see you cant paint over sanded 400 grit...PERIOD... I don't care what you are painting... I have yet to meet an old timer who needs 600... I use 320 and 400... Even if you are using candys you are going to lay down a ground coat.

A good friend of mine uses 600 all the time and thinks it is nessecary... I don't use it and you never see any sand scratches in my clear..The lower the grit the flatter the panel and the easier it cuts......

Guide coats are not used enough by the hobbyist... IF you sand down a car with 320 and a guide coat and get every bit of it off, you wont have issues... Of course this is my opinion.... You have to cover good with your base.. You should not be spraying tack coats with bc/cc...


Lastly in my rant... Times change and so do products... Back when I started painting, I was always told never to spray clear over any lower grit than 1000.... at that time the clear I was using would not feel those scratches I was informed... Now days you can re clear over 320... Some painters and I think my friend is one of them, hold onto some of the old ways that don't apply today..... If you don't put on enough base because you are a tack coater then you may see scratches... That is not the products fault... it is operator error... but most will say I should have used 600, not realizing they are not applying the base correctly...Same with the clear... same with gun adjustments etc

Barry or Brian feel free to correct me
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Old 11-26-2005, 11:53 PM
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To tell you the truth, I thought those short times on the Martin Senour products had been removed from the tech sheets long ago. Being three days isn't an issue in 99% of the time I never even thought about it. I remember when it was only a few hours after wet sanding!

Because of those early days of the product with a few hours recoat time I have always ran a gray scuff pad over it after a day or so. I figure it is cheap, very cheap insurance.

Now, why it happens. Boy, this is just a guess but I believe you are creating a fresh "skin" that is going to kick any uncured isos with the wet sanding being they are moisture driven. Just my hunch.

Brian
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Old 11-27-2005, 05:26 AM
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What the manufacturers were worried about is after sanding the primer the final solvents make a mad rush out for final curing and than if this was done with water it could accelerate the cure. Thats why they gave you a different
drysand and wetsand time frames.

You would still have adhesion with the base but not as good as you would if painted in their recommend time frame.

I have always suggested paint within 24 hours of final sanding or hit with scotch pad.

Truth be known most likely any of them would be fine a week later as long as car did not leave shop.
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Old 11-27-2005, 07:50 AM
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I have always thought it was overkill myself but I have always done it. My wife threw away all of my old printed copies of the martin senour tech sheets and I was looking at them on line the other day and noticed that the time between sanding and painting as well as no statement about wet or dry differences has changed from what I remember being stated on the sheets I used to keep as a reference. The way I remember the sheets it was something like 24hrs for dry and only like 4Hrs or something for wet which I always thought was an impossible deadline to spray if their was masking involved. Since the sand to spray wet times "were" listed as so short I always have scuffed before painting. I would like to skip this step simply to eliminate any more dust. Just never really understood the reason for the varying times? I wouldn't think that primer would unsand itself. Another thing that you will notice is that when dealing with the budget lines (crossfire, kirker) the tech sheets are much less detailed even though IMO the products are very similar. I have always tended to use the detailed sheets from the Tec line as my guide for the crossfire line when it comes to all the additional info that is missing from the crossfire 1 page tech sheets.(ex. crossfire will just tell you to sand. Tec __ will tell you do this, this, this, and if you let this happen do this.) It just goes to show you that the tech support is one of the biggest things that you are paying for with the higher end materials.
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Old 11-27-2005, 08:02 AM
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Another thing is you can bet if a manufactures states a time frame of 24 hours there is a safety net built in.

Another thing to keep in mind, is they are basing this on 70-75 degrees so if cooler or hotter that changes things- just another reason to build in a safety net.

When I did the 55 about 4 years ago the front end was epoxied with the rest of car and put the front end in wifes garage.
When I got around to fixing the front I did not scuff or re-coat the epoxy.
I applied the few spots of filler that were needed and than shot the primer over it.
Just my little test but than sold car so told new owner what I did so if he ever had a problem I would fix.
Still see car about every three months and that first thing I check.
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Old 11-27-2005, 08:08 AM
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By the way martinsr those sheets probably have been different for a while. The ones I am referring to were probably around 8 years old. Since the product numbers haven't changed I never really thought of going online and checking for any updates. Honestly I haven't used them at all for several years just continued doing what I remembered the tech sheets stating. I recently started using allot of kirkers primers and the sheets on those are vague. I figured I would reread the tec prime sheets and noticed that they seemed to have changed their views. I am like you the grey scuff pad has always seemed like cheap insurance and that is why I have always done it. Just curious as to what everyone else was doing in this regard.
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Old 11-27-2005, 08:39 AM
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Bondoking I agree that the guide coat is a good idea for a hobbyist. It makes it much easier for them to see the work that is being done. I myself rarely use a guide coat. As long as I am not shaping filler / primer with my fingers I can generally look at the condition of the surface being sanded to find high or low spots. The biggest advantage I see in using a guide coat is when switching from one grit paper to another to ensure that the coarser scratches are being removed. When I do a restoration I don't try to sand the same surface (coat of primer) over and over with varying grits and then go straight to paint.
My steps in a restoration are:
strip to metal, repair rust, apply filler, sand filler with 80 grit,
apply metal glaze over filler, sand with 180,
da sand all bare metal with 80 grit,
metal prep (not over glazed areas),
spray 2 coats of epoxy followed by 2 coats of 2k,
block sand entire car with 180 grit,
fix any lows or highs that show up during this sanding,
da sand entire car with 320 dry followed by hand sanding jambs w/ 320,
Spray 3 coats of 2k thinned down as a sealer,
hand wet sand entire surface with 600,
while wet sanding sheet water over entire car and check the reflection in
the panels,
paint
For me I find that by spraying the car with primer converted to a sealer after sanding with 320 allows you to really see what you have before painting. By sanding this coat with 600 it allows you to check every inch of the surface for sand scratches that were missed taking the place of the guide coat.
Is this the only way to do it? NO. But it works for me. Most cars I do will have only two priming sessions.
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