Metal Prep

THE most important part of any project, is prepping the metal properly.  It is the foundation that everything is built from, and it needs to be done extremely well for the paint job to last.  There is no ‘good enough’. There are two different ways to approach how the metal should be prepped prior to body work.  One route is to do the body work over an epoxy sealer on the metal. The second route is to clean and prep the metal properly, and do the body work directly on the metal.  Of course, either of these options can be done correctly and last a long time, or incorrectly.

The advantage of epoxy, is that the bare metal is sealed from the environment right away. It can be a barrier between the metal and any moisture that is absorbed through the body filler.  A quality epoxy sticks very well to metal, and filler can stick very well to the epoxy.

The ‘down sides’ of using epoxy under the body filler are for one, it takes a lot of extra time to do it properly.  The panels need to (should) come off the car to prep and spray them. There are times when the body is already assembled and adjusted, and this would be a lot of extra time, on top of the prep time.  There is a lot of additional time involved with prepping the metal properly for epoxy. The epoxy must be thoroughly dry (several days) before filler goes on top of it. Epoxy is a very slow drying product, and body filler (or polyester primer) does not like moving.  If the epoxy continues to dry under the filler, it will made the filler delaminate from the epoxy, and probably will crack. This means that the epoxy must then be scuffed and sanded as well. When doing body work, there will be break throughs down to the metal, and filler put back on top of it.  This means that there will still be filler over bare metal.

The advantage of doing body work directly over bare metal, is it is a lot faster, the vehicle can stay together once it is adjusted and ready for body work.  The body filler also sticks very well to bare metal. The possible ‘down side’ of body filler directly on metal, is that it is possible, if not careful, for moisture to make it’s way through the body filler onto the metal.  If you are careful, and aware of the environment, this shouldn’t be an issue. The primer, and especially the paint, will seal the body work and metal anyway.

These prep steps are very very very important!  Make sure every step is very well done, and that you pay a lot of attention to what is going on, so nothing is missed.  




A Basic Rule:

    This is a rule of thumb, that applies to prepping for epoxy, prepping bare metal for filler, prepping the epoxy for body filler, and a number of other things.  The rule is that to clean something well, it is best to start with a finer abrasive, and work your way down, getting coarser, until you get to the grit you need the substrate prepped in.  The idea of this, is that if something needs to be in 80 grit, for example, just sanding it with 80 grit, can miss a lot, since the grit itself on the sand paper is so much coarser and further apart.  So if you sanded something first with 150, then 80, the 150 grit would ensure that the metal is cleaned much better than if you used just 80 grit. Again, this applies for a lot of things, and is always a great thing to keep in mind when prepping something.


How to using Epoxy:


These steps are assuming that all the welding, adjust, and any other metal work is complete, and the car is ready for body filler.  This is also assuming the car has already been stripped down to bare metal in a previous stage of the build.

  1. Clean!  The first step is very important.  Clean the metal very well by rinsing it and wiping it with lacquer thinner.  This will be sure to get any oil or sharpie marker out of corners, or small weld voids, or anywhere else you can’t tell it’s contaminated.  There are a lot of times that the metal looks clean, and as soon as lacquer touches it, all the sharpie comes out. If the metal is clean right away, it will be a much better start and foundation to everything else.
  2. Sand Blasting.  The entire car does not need to be sand blasted.  The only areas needing blasting are any rust pitting left over, any weld seams, and any sharp inside corners.  The corners, are only to help with adhesion for the epoxy, since it isn’t always as easy to get sand paper in these corners, so the blasting will help ensure there will be something for the epoxy to hold on to.  The pitting and welds should basically be seen the same way. They both need to be “whited out”. This isn’t always easy, but they both need to be clean. The pitting cannot have any dark coloring in it, that just means the rust is still there.  It needs to be completely white and clean metal all the way to the bottom of the pit. The welds must be the same way. If the seam is completely smooth, with no voids at all, it probably isn’t necessary to sand blast it. Any weld seam that has an little bit of voids, or anything else in it, needs to be blasted.  Nothing properly sticks to welding coloring or porosity.
  3. Epoxy should not be applied to sand blasted metal; at least not with the caliber of work we are aiming for.  There is far too much dirt left behind from sand blasting, and it’s not worth that being the foundation of all the body work and paint.  So, once the sand blasting is done, the metal is ready to be cleaned.
  4. Step one of cleaning the metal, is to use a red scotch brite (The rule of starting finer, and working down) and a metal cleaner.  Be sure the metal cleaner is not and acidic based cleaner! We are not trying to etch the metal. Glasurit makes a very good metal cleaner, that doesn’t leave anything behind.  Scrub all of the metal good with the scotch brite and metal cleaner.
  5. Once the metal cleaner is done, clean the panels with a solvent based wax and grease remover and towels, to make sure all the cleaner is off the metal.
  6. Using 150 grit and a D/A, sand the panels very thoroughly.  150 grit does a great job of cleaning, as well as making sure there is at least 150 grit for the epoxy to stick to, incase anything is missed with 80 grit(which is next)
  7. With 150 complete, move down to 80 grit on a D/A.  Be sure that the paper is sharp, you want to make sure there is plenty for the epoxy to bite onto.  
  8. With 80 grit complete, it’s time to clean again!  Do not use lacquer to clean the metal.  Lacquer thinner tends to hold/absorb moisture in it, and there is a chance that it will leave a small amount of moisture on the metal, leaving small rust droplet spots.  
  9. Clean very well with wax and grease remover.  Spray the cleaner on the metal, and wipe clean.  Repeat this until the towels stay completely clean.  Yes, this is possible.
  10. Once the panels are all clean, they are ready for epoxy!  
  11. Spray two medium-wet coats of epoxy, following the tech sheet for the proper flash times. It should probably be at least 1-2 hours(or more) between the two coats, this will help make sure the epoxy is able to dry enough.
  12. After the epoxy is sprayed, it is very very very important that it is completely dry before body filler goes on top of it.  A good infrared lamp is the best way to speed up the cure times. It should still go through several long heat cycles, like an hour of heat at a time, and 3 or 4 times.  If there is no infrared lamp, bake the epoxy in the paint booth, with several bake cycles, and allow the epoxy at least 2 days to sit, even with the bake cycles.
  13. Once the epoxy is dry, it needs to be prepped for filler.
  14. Scuff the epoxy with a red scotch brite.  Don’t get too crazy, it just needs to be gone over.
  15. After scuffing, 100 grit, or 80 grit, can be used by hand to scuff up the epoxy as well.  You have to be careful here, the sand paper pretty easily cuts through the epoxy. Lightly sand it just enough to make it look like it has all been sanded.  
  16. Blow off and clean with wax and grease remover again to get rid of sanding dust.
  17. Make sure the wax and grease remove is completely dried out before using filler.  I would recommend allowing at least a couple minutes, just to be sure.
  18. Filler time!


How to over Bare Metal:


These steps are also assuming that the vehicle is already in bare metal from a previous part of the build.  It also assumes that all metal work is completed, including welding and adjusting panels. For this route, it is possible to leave all the panels on, and not worry about the jams and some other areas until later on.  Judgement needs to be used for this. If the bottom, or jams, or engine bay need a lot of blasting, it might need to be done right away. The next chance will be when the car comes apart for the paint process. I will assume, for this purpose, that everything (floors, engine bay, jams) is clean, and not needing much prep work.

  1. Clean!  The first step is very important.  Clean the metal very well by rinsing it and wiping it with lacquer thinner.  This will be sure to get any oil or sharpie marker out of corners, or small weld voids, or anywhere else you can’t tell it’s contaminated.  There are a lot of times that the metal looks clean, and as soon as lacquer touches it, all the sharpie comes out. If the metal is clean right away, it will be a much better start and foundation to everything else.
  2. Sand Blasting.  The entire car does not need to be sand blasted.  The only areas needing blasting are any rust pitting left over, any weld seams, and any sharp inside corners.  The corners, are only to help with adhesion for the filler, since it isn’t always as easy to get sand paper in these corners, so the blasting will help ensure there will be something for the body filler to hold on to.  The pitting and welds should basically be seen the same way. They both need to be “whited out”. This isn’t always easy, but they both need to be clean. The pitting cannot have any dark coloring in it, that just means the rust is still there.  It needs to be completely white and clean metal all the way to the bottom of the pit. The welds must be the same way. If the seam is completely smooth, with no voids at all, it probably isn’t necessary to sand blast it. Any weld seam that has an little bit of voids, or anything else in it, needs to be blasted.  Nothing properly sticks to welding coloring or porosity.
  3. A 3M clean & strip disc is the first step in prepping the metal for filler.  These discs do a great job of cleaning, and the don’s miss anything; they create a very smooth and even surface.  This can be used on a 3” grinder. Be careful not to use too much speed, creating too much heat into the panel. It is fairly easy to heat the panel up too much.
  4. Clean the metal with wax and grease remover.  The metal won’t be able to be wiped down after it is ground with a roloc, so this is a good chance to make sure the metal is clean, since it’s still smooth enough.
  5. Grind the metal, carefully, with a 3” roloc.  They type of roloc will determine what grit to use.  The goal is for it to feel like the metal is almost fuzzy from the grinder.  If it still feels smooth, the roloc isn’t sharp enough. I prefer the 3M purple cubitron roloc, in 60 grit. These last a lot longer than most grinding discs, and the grit is so much sharper, it grinds the metal nicely.  For the 3M green corps discs, I prefer 36 grit to prep for filler. This line isn’t quite as sharp, so it takes a little bit lower of a grit to grind the metal well enough. Make sure to pay attention that the roloc is sharp enough.  It will continue to look the same, but the metal is actually smooth. So, just always check and feel the metal to make sure. This step is important, it’s what everything will be hanging on to.
  6. Blow the metal off very well, and it is ready for filler.  Don’t use wax and grease remover here, since the rag will stick to the metal too much and make a mess.  

6 comments

  • Hola, en primer lugar queria felicitaros por el grandisimo trabajo que haceis y por compartir estos consejos, para personas como yo nos es de mucha ayuda, inspiración y ganas de hacerlo cada dia mejor, mi pregunta es, ya he trabajado el relleno siempre aplicando epoxi en metal desnudo, ahora aplico una capa de epoxi encima del relleno antes del poliester a pistola,lo dejo secar en cabina a temperatura de 25°,¿cuantos dias debo dejarlo secar antes de aplicar poliester a pistola? Muchas gracias por todos los consejos y la fuerza que me dais para seguir con estos grandes proyectos, un saludo

    Agustin casado
  • Scott. Why does it create moisture? I’ve never once seen this happen. I’ve seen bad prep work, but I’ve never seen moisture created from body filler drying. Just because something gets warm, doesn’t mean it creates moisture. The metal would have to be under the dew point temperature for that to happen, which it will not be. That’s why a coffee cup doesn’t sweat. It is completely about the quality of your prep work.

    Tyler
  • Here’s the problem I see with doing body work over bare metal.
    During the reaction process of the body filler drying it will create moisture trapped between filler and bare metal. Rust!

    Scott
  • Well done again. Good in depth advise. Good to see you covered both systems. I prefer the epoxy method but each to their own. The Glaz metal clean is a great product. Keep up the great work. Looking forward the the pre polyester primer steps with the bare metal approach. Z

    Ziggy
  • Real nice Tyler… I like that you thoroughly recommend steps for both procedures. Micro-analysis and conversation in this area of a build is an arena few can truly appreciate or engage in. Thank you for taking the time to share your experience and insight on the topic.

    F-Man

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