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Frequently asked questions about urethane primers

Automotive undercoats, primers, sealer and body shop supplies available to order online.

We have compiled the most frequently asked questions about automotive undercoats. We will continue to add more of your questions as needed.

Undercoats, Primers, Sealers and Plastic Coatings

1. Question: My car is stripped down to bare metal, what should I use to help adhesion to the metal?

     Matrix MP-550 Self-Etch Primer is a two-part primer that chemically protects against the corrosion process. Recommended for protecting bare metal, galvanized steel and aluminum, MP-550 gives medium film build to help eliminate imperfections and can be lightly sanded.

Surface preparation: Remove tar and grease with Wax and grease remover of your choice before sanding. Sand surface with 180 to 220 grit dry and re-clean surface. If metal surface is badly pitted or rusted, the rust must be removed. Remove by grinding or sandblasting until all rust is removed.
Aluminum and stainless steel MUST be sanded with 150 to 220 grit dry sandpaper even if a metal prep or self etching primer is used. Prime aluminum and stainless steel within 72 hours, and prime bare steel immediately after cleaning.

Mixing: Mix 1 Part MP550 Primer with 1Part MA505 Activator.

Application: Apply 1 to 2 coats, wait 30 minutes and then apply MP2K Urethane Primer. 

Order MP-550 with MA-505 Activator Now!

2. Question: What is the difference between Lacquer Primer and Urethane Primer?

    Urethane primer is about 50 times better. Many shops have not realized that lacquer primer is the most expensive primer to use in the market. All though the price tag is an attractively inexpensive cost initially, the side affect causes a transparent high cost and many shop owners never even notice.  The number one cause of unsatisfied customers after experiencing an automotive finish repair was due to the use of poor quality undercoats. Lacquer primer is made of talc and resin and is categorizes as a thermoplastic type of paint resin. What is thermoplastic? The simplified answer is thermoplastic products can usually become a liquid state again.

     For this example, let's say we apply some lacquer primer on a surface and let it dry (notice I did not say cure) outside for one week, a month, or even 2 years. We pull the car inside and take a razor blade and scrape off the primer and put it in your mixing cup. We pour some good grade lacquer thinner into the cup and wait a couple of days. Stir it and it will be become a liquid state where again, and depending on the resin that was used, you may be able to apply it back on the car.

     This will not happen to urethane primers. Urethane and Epoxy primers are activated and go through a thermosetting stage. Again, to simplify the answer, Thermosetting in this circumstance is an activated resin that is used in urethane primer becomes a plastic type finish. The new coating structure cannot go back to it's original state. It is designed to become a strong, durable, chemical resistant coating.

The benefits of using urethane or epoxy primers are endless. The body shop and consumer are rewarded by having no lose of gloss, no shrinkage or swelling, delaminating or cracking. The solvents that are used in the basecoat systems are very strong. You must have an activated and cured undercoat in order for it to resist and repel the solvent from penetrating into it.

     Since lacquer primer never cures and is made up of talc and thermoplastic resins, it absorbs the solvent instead, therefore causing lose of gloss to the topcoats followed by paint swelling. And if your lucky that's all. In more severe cases the topcoat will delaminate off the lacquer primer.

     Urethane Primer contains more solids than lacquer primer and you will use about half the amount to do the same job.

We recommend you use the MP-2K Urethane Primer. It has very easy to sand properties, high build and cures fast. Mixes 4:1 with activator and does not need reducer.

Order MP-2K Primer with Activator Now!

3. Question: How much urethane primer should I order?

    This is a tough question because there are too many variables, The type of paint gun and fluid tip size will determine the usage in a big job. Other factors vary depending on how much body filler is on the car. This may require more coats than other panels. But for the most part, a medium size car should make it with about 3 quarts to a gallon of MP-2K Urethane Primer.

Order MP-2K Primer with Activator Now!

4. Question: I use urethane primers and I keep getting swelling marks in areas that I used Body Filler and Putty. What is causing this problem?

     There are several reasons that can cause this but let's break up the question into two parts because you may have two different things going on.

     I was recently in a client's body shop that was having a similar problem. I identified three types of swelling problems and solutions:

     One type of swelling is from the 80 grit or even 36 grit scratches that were not taken out completely. They look like straight lines embedded in the paint film. After doing some research and observing the process of the prep techs, I discovered that the prep person thought he had "Primer Bondo" in his gun and tried to fill in the scratches with the Urethane Primer. There is a big misconception of the label on these primers that say "Extra High Build Primer" or "Ultra-Magic Fill" and makes it sound like there is a miraculous ingredient in the can that is going to fill all the scratches left on the body repair.

     The solution to this problem is to finish out the body filler with 180 grit and fine polyester putty, This will speed up the process in the paint shop, save materials and increase the quality of the repair. But in this particular shop like many others around the country, the prep tech hoses the body filler with primer and eventually fills the scratches. You can immediately see the sand scratch swelling in about 2 to 5 days depending on the temperature.

     If you ever had a chance to read tech sheets for urethane primers, most manufacturers mention to finish off the body filler or polyester filler with 180 grit -  220 grit sandpaper. Some may even go as coarse as 120 grit. But I have seen better results with the shops finishing off with a fine polyester putty and 220 grit finish. The other important factor is the flash off time between the coats, I know that sounds like a simple step but I physically stood and watched this prep tech apply a coat primer every 30 seconds on all four coats. All this time he though he waited 5 minutes between the coats. If you don't let the primer flash off at least 5 minutes between coats you may be trapping solvents underneath the primer and the solvent will start penetrating into the body filler softening the paint feather edge and swelling the body repair.

     The second type of swelling is the markings of a map. Usually where body filler was used or a "Bust Through" on the paint finish has occurred during sanding. The feather edge is exposed vulnerable to solvent attack from the topcoats. It looks like a broken circle with fuzzy edges and you notice loss of gloss in the repaired area.

     The main cause of this is heavy wet coats with very little flash off time, not enough back sanding before applying the primer, it is recommended to use 220 grit sand paper, avoid scuff pads for back sanding, this will not dull out the finish completely. If you have to use a scuff pad then use sanding past in conjunction.

     Another factor is the curing time of the body filler or putty. The average body filler at 70 degrees cures at 16 hrs. Raw body filler will also cause loss of gloss and swelling. If you like in a cold climate, try using heat lamps to kick off the chemical reaction.

     The third is there may not be enough primer covering the "bust through" feather edge, always apply primer at least 4 to 6 inches past the feather edge. This will allow you to block sand the primer and still leave enough film build to protect the feather edge from solvent attack. If you use a DA sander with 80 grit to feather edge, be sure to finish and completely dissolve the 80 grit scratch with 180 grit or 220 grit. Do not attempt to fill the DA 80 grit marks left over because these half circle marks will also swell.

5. Question: What kind of Spray Gun should I use to apply Urethane Primers?

     This is an ongoing change as spray gun technology changes. In regards to automotive body repairs, you should be using a gravity feed gun with a 1.4 mm fluid tip. Some manufacturers may recommend a 1.8 mm to a 2.0 mm fluid tip depending on the solid content and viscosity of their primers.

     It has been my experience for the last 15 years as urethane primers have evolved. The best results were established when the smaller tip sizes were used. The advantages that I saw were faster flash off times, faster cure times, less runs per application, less material wasted and better gloss hold-out.

     The larger tip sizes were hosing on too much primer, even if you wait to apply the next coat, the top would skim over and trap solvents underneath. This would later cause shrinkage, swelling and loss of gloss. The other problem I saw was too much material would accumulate in body creases, eventually causing bubbles along the middle of creases, natural body indentions, around door handles and moldings.

6. Question: How many coats or mils should I apply?

      I apply one medium coat first, then apply 3 wet coats over a repaired area and 2 to 3 wet coats over normal non-repaired surface areas. I don't think you should have to apply more than 4 wet coats of primer over a repaired area. If you need to apply more coats, I would stop, let the primer cure for 24 hrs, block sand with 220 grit and re-apply the following day. If you follow the steps in Question 4, you should not have to apply more than 4 coats at any time. Fix your imperfections with polyester body filler first before top-coating primer. Variables may include gun stroke speed, fluid tip size, use of different air pressures, is a fast double coat = one coat, and so on.

      You should have at least 3 mils of primer left over after block sanding to protect the body filler or busted feather edge from solvent attack. One coat of primer yields about 2 to 3 mils per coat.

     What is a mil? A unit of length equal to one thousandth of an inch. Unless you have a mil thickness gage to determine the thickness of your paint film then it is of no value to you. A plastic wrapping that seals a cigarette box is approximately one mil thick. A new car from the factoring has 5 to 7 mills of coatings that include galvanized coatings (0.5 mil), sealer (1.5 mils), Basecoat color (1.5 to 2.0 mils) and clearcoat (2.0 mils to 4.0 mils)


Always apply products according to the manufacturers recommendations.

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