Bare Metal Prepping/Cleaning for Primer

  • Thread starter Bob Hollinshead
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Bob Hollinshead

Some common mistakes I've seen people make is not prepping bare metal properly for good primer adhesion. Keep in mind epoxy and DTM type primers need the metal to have some texture and it also needs to be perfectly clean. The perfect texture is achieved by sandblasting or sanding with 80 grit on a DA, some fiber discs like 3M's Clean-n-strip discs also work well. Failure to texture the metal and clean it well can lead to less than adequate adhesion. Blasting jambs and similar areas is usually no problem but outer sheetmetal should be left to a professional to avoid warpage-(Shine should do a thread on blasting techniques). Hand held gravity fed spot blasters are an economical way for anyone to clean small areas-they work well. When sanding change your paper often so it produces a sharp scratch pattern and not a shiny polished surface.

Clean the bare metal with solvent based wax and grease remover then follow that with waterborne wax and grease remover-spray the cleaner on and wipe it off while it's wet, or you can pour the cleaner on one towel then wipe it on and wipe it off with a clean towel. Why two cleaners?-they each offer their own benefits. (Update: New SPI guidelines call for waterborne to be used first, then solvent base.) Should sandblasted metal be cleaned-definitely and especially if you recycled your sand. I've seen primer fall off of sandblasted steel when the person didn't clean it before priming. You'll end up with some towel fuzzies on a blasted surface as the texture grabs the fibers-no big deal just give it a light scuff with a red scotchbrite and a blast with the blow gun and they are gone and they need to be gone!

Simple Green, 409, Dawn dish soap, Ajax, Fast Orange, Engine Degreaser????-- these can all be handy during the precleaning stages of a job but you shouldn't be cleaning your bare metal with them for the final clean before primer.

Will a wire wheel work for texturing the metal-No they mostly provide a polished or burnished surface, they do work well for rust removal though.

Will an acid etched surface provide enough adhesion? No, if you use acid make sure to neutralize it well and then mechanically texture the surface with sanding or blasting. Acid works great for rust removal but needs to be completely removed for any primer application.

Safety concerns? you need to understand the safety concerns for all autobody products and tools-every one is dangerous in one way or another.

Feel free to add or modify as needed.
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That was just perfect, thanks.

I have a lot of shops that do clean the metal with 710 first and then the 700, like you say, that is the perfect way, perhaps when I talk to people I should stress this way but I have tended to neglect mentioning the two steps and really should offer that advice more.
This is where I'm at right now. The car is 80% blasted and I've been thinking about the prep before primer. I was going to blow off all the sand that I could with a blow gun, wash the car with warm water, Dawn soap and a red scotchbrite pad, and then follow with waterborne w/g. Thanks for the info, now I'll hit it with solvent based w/g also. What type of towels should be used?
I use my electric weed blower to dry the car with, never thought about using to to defuzz it before primer! ( is defuzz a word,lol)
cjetmech;5616 said:
This is where I'm at right now. The car is 80% blasted and I've been thinking about the prep before primer. I was going to blow off all the sand that I could with a blow gun, wash the car with warm water, Dawn soap and a red scotchbrite pad, and then follow with waterborne w/g. Thanks for the info, now I'll hit it with solvent based w/g also. What type of towels should be used?

Any nonprinted towels, your basic kitchen style paper towel will work, or you can spend some money on lint free type towels designed for prepaint cleaning. The cheaper towels will leave fibers that you'll have to deal with. I've seen people have fisheye problems using printed paper towels.
Thanks bob. Whats the general opinion on washing with soap and warm water? I've heard others like shine say its no big deal. I was going to knock off any flash rust with a scuff pad when I go over it with w/g.
I was thinking that if you do wash with soap and water, a gallon-jug or two of distilled water from the store might be in order. Tap-water has so many 'chlorides' in it its closer to seawater than anything else. I'm exageratting obviously, but you get my drift, it's isn't just the oxygen in the air that creates rust, but also the salt or chlorides in the water used, which are plenty in tap-water.

In the power-plant business, water isn't the enemy of iron, but any kind of salt is, in a big way...
In my garage I like the "Box of rags" you get at Lowes or Home Depot. They are real heavy paper type towels in a box of about 250 I think, and cost about $10. I use them for everything. They are great for use with W&G remover and even washing the car. Cheap enough that you just throw them away. Usually once I put one down it is going in the trash. Only way I can make sure I don't wipe the wrong thing on a car or me.

i wash everything. there is no better oil dispersant than dawn soap . orange flash rust is usually iron deposits in your water. it hurts nothing and will cause no problems. after rinse i blow dry with a leaf blower and get very little flash anyway .
Shine, I agree the orignal style Dawn dish soap is a good basic degreaser but the mineral and waterborne cleaners will take off other contaminants. If you sand a panel and wash it with mineral based wax and grease remove then folow up with waterborne you'll see the wipe off towel still gets dirty on the waterborne cleaning-the waterborne pulls more off. You can do the same test after the Dawn cleaning. Some people recycle their abrasives and this ends up hammering contaminants into the metal-contaminants like undercoating, road tar, motor oil, gear lube, adhesives, sealers, wax... for these types of contaminants the solvent based cleaner is a good first step and Dawn dish soap isn't good at removing road tar, undercoating... JMO
nothing goes into my blast room until it has been soaped and power washed. i want no grease or oil in my booth. i do not like mineral cleaners on bare metal. metal is porous and when you dilute and melt contaminates it can stay in the metal. much like washing fiberglass with thinner.
I could not agree more about degreasing before blasting. I usually spray everything with Purple Power (much stronger than 409 or Dawn) and pressure wash. I repeat as many times as needed to get everything grease free before blasting. I've used a heat gun and a scraper to get off rubber/tar like coatings. Contaminating the sand will make it less effective, and like Bob said, you are impregnating that contaminate into your metal.

With that said, is anyone recommending washing freshly sand blasted metal with water? I've heard of using water on soda blasted substrates and other less abrasive materials, just never sand.
first if you use sand everything you did was wasted effort.
and yes i wash every car after blasting.
What blast media do you normally use Shine? What problems will I have if I use sand? I use really fine white sand. I get it from a friend of mine who has a mobile blaster. I have literally used tons of it without any problems yet.
any type of quartz has caustic salts in it which promote rust. i use olivine or starblast. this is what i learned researching before investing in the blasting business. i prefer the acrylic to remove paint then the others to remove rust.
Thanks Shine. What are the chances I will have a problem down the road with jobs I have done this way? I always clean it really good with waterborne.

Also, when you get a minute could you please look at my post on my 1934 Chevy. I am wondering if the front suspension I took out of it is Corvette.
it usually shows up around overlaps and seams as a light rust. no holes but rather like surface rust. extreme case maybe a bubble.
I have seen light rust at seams before. I always thought it was just that I didn't get paint in the seam as far as what the sand cleaned. Can you recommend anthing for these areas? I have blasted a lot of stuff with sand. I cringe at the thought of it eventually rusting.

I do a lot of blasting on farm equipment too. I normally borrow my friends Sullair portable blaster. Do you think I can get away with sand for that kind of stuff....I basically get the sand for free and I go though a lot of it.