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MG MGB Technical - Paint tips needed: When to use sealer

I'm planning a respray of my car soon. It's stripped to the bare metal right now and I'm in the process of pounding and filling dents. I'm using Bondo as a filler. My question is, when do you use a sealer? I've primered right over Bondo in the past with a sandable primer and wet sanded. Then I topcoated with an enamel finish. The problem was when the paint dried it swelled the sand scratches in the Bondo beneath and made them stand out really bad. The scratches were not visible after wet sanding the primer. I was told that this was because I failed to use a sealer. When is it applied? Right on top of the bare metal and Bondo, or on top of the primer? What if I used a faster drying paint like laquer instead of enamel, would that have swelled the scratches just the same?

My second question is about epoxy primers. If they are difficult to apply and to sand, then why bother? There must be some benefit to using them, otherwise everyone I know would stop telling me they used them on their cars!

ANY advice appreciated.

Thank you so much.

Scott
Scott Wooley

Scott

I never found Bondo to swell, but I don't use the stuff anymore. When sanding Bondo, do you see small pinholes? Those pinholes are throughout the filler, which can lead to rust where it contacts bare metal. Bondo has many uses, but automotive bodywork isn't one of them.

I would highly recommend a good polyester filler like USC Blue Ice or Evercoat Easy Sand. These are high performance fillers that sand easy and give professional results. Very helpful is the use of Eastwood's 14" flexible sanding block (and paper) for long runs of metal. I used to wet sand, but my local auto restoration shop recommends dry sanding only. Again Bondo is a poor choice here because it will clog up the sand paper. Use a good dust mask when dry sanding.

My bodyshell was beadblasted, then painted with Glasurit epoxy primer. Duraglass filler was then used over previously welded seams, USC Blue Ice to finish off the car. I don't spray epoxy paint because of the toxic problems. Sanding epoxy primer isn't hard, use a good quality paper, working up to finer grits.

I'm still applying and sanding filler, but future plans are to again spray the car with Glasurit epoxy primer sealer over the filler, dry sand smooth with a fine grit paper, then apply Glasurit epoxy top coat.
Richard Stinchcomb

Put the sealer on over the primer, after you've wet sanded it. The matte finish of the primer conceals the minute scratches left from sanding.
Applying a coat of glazing compound ( I guess this is what you're referrring to as sealer)fills any scratches from the first sanding. You then sand this coat out with fine wet/dry paper.Bodywork is a crappy/messy/ time consuming process in my opinion, which is why that's the part of the job I turn over to the pro's, along with the painting. If you've got sanding scratches, you're probably not using a fine enough paper for you final sanding.
R. L Carleen

Scott,
If you have trouble finding any of this stuff, there is a place in Denver on Santa Fe and Hampden (3701 S. Santa Fe) that is a VERY good paint supply place for auto paint. Painters Supply (fancy name, huh) at 303-762-1606. Expect they can offer pretty good advice, too. They even have the old paint charts (BL, BMC, etc.) and cross-checked the color # when I had them make up some spray cans to do my engine compartment. Some of the guys are more knowledgeable than others so ask who knows the most about it. Lot of the good auto body shops buy there.

JTB
J.T. Bamford

Thanks for your comments. By sealer, I'm talking about something you spray over the entire car to "seal" whats under the sealer from coming in contact with what you apply on "top" of the sealer. I'm all too familiar with glazing compound, using Bondo as a filler! A sealer is supposed to prevent the solvents in your paint from soaking in to the layer down below it and reacting with it, or in the case of Bondo, swelling fine scratches into more visible scratches. It doesn't happen all the time, but it can happen, I know. Trust me when I say there was no sign of any scratches before the top coat went on. As far as wet sanding, I was refering to the primer layer. I've never heard of dry sanding only the primer.

Thanks.

Scott
Scott Wooley

Scott, My PPG distrubutor mixed up some sealer for me which was the same color (BRG in my case)as the final paint I was using on my 67 B. The Sealer was applied just before the finsh coats of paint were applied. The sealer was a light film applied to seal the bodywork/primer from the finish paint. It was mixed to the color of the finish coat as a technique to assure coverage and to stand out from the primer underneath it. The sealer was scuffed with a scotch brite pad prior to applying the final paint. Talk to your local paint experts (I used a PPG Omni system) and be sure the paints/sealers/primers you use are all compatable. This was my first try at bodywork and painting and I learnd a lot. Next time around it will be easy. I recommend PPG products if you have a supplier in your area. The biggest thing I learned was the steps involved and the basic products (abrasives,fillers, primers,sealers,paint, etc.) to use in each step.

Good Luck

Moe

Moe

It seemed that you may be confusing glazing putty and bondo. Perhaps I misread your statement. Glazing putty is used after the bondo to fill pin holes and to give a better, thinner edge. If you have not used some sort of glazing putty you will always have imperfections in the final product because bondo is more porous. I have always used green stuff but there are probably better products out there.
jim mindy

Sorry, I wasn't more clear. Bondo always has pinholes. Therefore, I'm familiar with glazing compound, which goes on afterwards to fill these holes. This stuff will do absolutely nothing to prevent the paint from soaking into the Bondo though. I think I have the answer I was looking for. Apply the sealer after the primer, but before the topcoat.

Moe, I can get PPG paint close to me, but the trouble is, the guy who works there and mixes the paint is not the most knowledgeable fellow I have ever met. I live and work near the Denver metro area, so finding a good paint store shouldn't be difficult. I plan on shopping around for one soon.

Thanks for all your comments.

Scott
Scott Wooley

Scott: Do a search on the internet for PPG paint or something like that. Or look in the phone book for a paint supplier. I called my local NAPA/Auto paint supplier where I buy my tires and asked if he would help me with my resto if I purchased the supplies from him. He said yes and helped me with basic info through the entire sanding, body work, priming, sealing, and painting. PPG is obviously not the only brad that will work. The key is to find a paint "jobber/supplier" that will help you and sell you the right stuff for the phase of bodywork or painting you are in. The guys helping me had 30 plus years of experience and were glad to help a new guy with a resto project....Hope that helps.

Moe
Moe

I'm going to attempt to paint my 73 rdstr soon, as well. I've done quite a bit of compressor spray painting, but never an entire car, so I've been searching for a step by step on the internet to ensure I have the correct information before jumping in.
I have come across a couple of different theories concerning "sealers".
One website says seal the bare metal prior to priming. This web site did not mention what exactly this "sealer" is supposed to do.
Many web sites mention sealing after the final color coat, of course this is clear coat.
However, another mentions sealing the primer so the color coat goes on smoothly. Again no explanantion of what type of sealer, nor what it does.

So far I have striped the fenders, trunk lid, and doors down to bare metal and simply sprayed the metal with a sandable Hi-fill primer. Sanding between several coats. I thought once the primer was smooth (wet sanded to about a 600 grit) All I would have to do is spray on my choice of color, then maybe clear coat.
Am I about to miss a vital step that will result in my having to do this over in a short time?
TDHoward

TDHoward,
I know exactly what you mean about one site saying one thing and one site saying another... I have been told by several sources that a sealer on top of primer is not necessary UNLESS the following applies:
(1) The car has Bondo under the primer coat. The solvents in the top coat can and will soak through the primer and into the Bondo, creating a color shift where the paint soaks in. There are body fillers that are more immune to this phenomenon, but I have no experience with them.
(2) You are applying a topcoat onto a chemically incompatible primer. (Enamel paint on lacquer primer for example) A sealer is necessary. It is always best to use primer and topcoat from the same manufactuer and designed to work together as a system.
(3) Any time you are painting over old paint, where there might be compatibilty issues or if you have no idea what kind of paint you are painting over.

There may be other reasons, but these are good enough for me. I'm going to use a sealer over the primer tinted to match the topcoat. I plan on using a laquer based sandable primer and a laquer topcoat. I'm not afraid of the color sanding or buffing that will be necessary. I've got lots of time...

Scott
Scott Wooley

This thread was discussed between 05/06/2003 and 12/06/2003

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