Thursday, November 28, 2013

New product review: Mastercoat Metal Prep

Some time ago, I was sent a goodie bag of refinishing chemicals to evaluate and play with.  One of them grabbed my attention in particular and has made itself a frequently referred to, permanent fixture in my shop. 

It is a phosphoric acid-based metal prep distributed by Mastercoat.  Much like Ospho only better, Metal Prep also contains a detergent for cleansing action of your dirty parts as well as a higher concentration of phosphoric acid.  Oh, and it's REUSABLE.  Its presentation is a yellow liquid and it is pretty potent stuff.  It has a sweet smell and some noxious fumes associated with it.  Personal protective equipment should involve safety glasses, gloves, and a respirator, which are standard fixtures in my shop.  Before I continue, I should include the disclaimer of always follow the tech sheet of any products you are using, including the metal prep, and especially other products that may be used in conjunction with phosphoric-acid containing metal preps.

This is especially great for cleaning up rusty bolts, nuts, clips, etc, to like-new condition in as fast as an hour.  I keep a small Gladware container in the shop with a few inches of depth of the Metal Prep.  When I need to clean up some bolts I just toss them in there and check on them in 30 minutes to an hour, if not sooner. 
Here I have some rusty clips soaking.

This is a 'before' and 'after' with a comparative sample.  The clip will go from rusty and grimy on the left to clean and like-new in under an hour.

These are the same rusty clips shown soaking above.  They now look new, and are nicely prepped for Parkerizing.

But this also has other ways I put it to use to besides soaking small parts.  It's especially useful for removing heavy rust in hard to get areas, especially where you couldn't fit a sander or a blaster.  For instance, on the inside of this door:
This was the inside of the driver's side door skin.

I wiped the area down liberally with the Metal Prep, then laid out SATURATED paper shop towels, smoothing them out as much as possible to make full contact.  I occasionally continued to add more solution to keep the towels wet.

After sitting overnight, I peeled the towels off the next day and the majority of the rust was gone.  The remaining rust you see was black from the chemical conversion process that occurs with iron oxide and phosphoric acid.  The paper towels were quite difficult to peel off the next day as they had mostly dried and then stuck to the metal.  After it was completely dried, I brushed on two coats of the Mastercoat Silver permanent rust sealer.


  1. Just (re)discovered your blog, Josh. Thanks for the heads-up on that Metal Prep. I did most all of my car with Ospho and though I soaked many a part in the stuff, never had such impressive results as this.

  2. I have never compared to Ospho myself, but I know others who looks like this still works better however. This has almost twice as much phosphoric acid than Ospho.

  3. Love this stuff. I found it based on your blog and it brought them some business. Now that I've cleaned up all the nuts and bolts, what did you do or use to keep the bolts and nuts and such from rusting again? Did you parkerize? If so, what kit did you use? Thanks.

    1. They won't actually rust for a long time. I did a lot of these bolts back in Aug/Sep and they still look good. If you rinse them off with water after the acid dip they will have flash rust the next day. If you're painting them, you can clean with wax and grease remover. I have not parkerized, I do plan on it, but here is what one of my uber anal friends does:

      I use the LCW or there is also the Lauer from Midway. You can buy either in the 1 gallon or 1 quart.

      They both work the same and you would really like the results. You will need a stainless steel pot, (like your wife cooks spaghetti in), a stainless strainer (for dipping parts out), a cooking thermometer, and a hot plate. The wife will be pissed if you do this on her stove. I bought a stainless steel pot at Meijer. Clean metal (bead blasted) and temperature are important with this.

      The solution mixes 4:1 (4 parts distilled water,1-part solution)

      1. Heat the mixed solution in a stainless pot on a hot plate to 170 - 190 degrees
      2. Once up to temp, through a ball of steel wool in the pot for about 20 minutes to get the process started.
      3. Remove steel wool and toss your "clean" hardware, steel parts, etc. into the pot.
      4. Cook parts at temp for 10 - 15 minutes (stir parts once or twice while cooking)
      5. Remove parts and rinse in clean water
      6. Blow dry parts and spray with WD40 and Let set for 5 minutes

    2. Thanks, Josh. I saw the postings on the other forum where you shared the Mastercoat information too. Keep posting on this blog. I stop by every once in a while to learn. I too am restoring a 67 coupe, although I'm not even close to your progress or experience level. Now I'm headed to thrift shops for this list.

  4. Thanks JS. We're all always learning!