Rust Sealer FAQ

MCT101 thinner or medium urethane reducer at 10%. You can also use mineral spirits for this product, but use less.

Yes, they are UV stable.

Yes, they go on nicely with a sponge brush or a 4 inch foam roller.

Silver is the color of both rust sealers, Original and Slim. When we first started in business, we only had rust sealer and the black chassis finish coating. We referred to them as the "silver" and the "black". However they're very different from each other--the silver is the much more effective rust sealer and the black chassis finish is better as a topcoat.

Mastercoat Permanent Rust Sealer is a moisture cured urethane. The more moisture in the air, the faster it dries so it’s actually strengthened by moisture. The aluminum powder in the coating cuts off the oxygen so the coating itself is waterproof and air tight. It’ll withstand 8000 hours of salt spray. This product was originally developed in the mid-70’s as a bridge primer. Bridges coated with 2 coats of silver and one coat of color such as our AG111 Ultimate Finish Coating have a documented 20-25 year service life and will take up to 14,000 hours of salt spray.

Mastercoat Permanent Rust Sealer is sunlight stable, sandable, paintable, sticks to clean steel, and is air tight. We don’t recommend painting just black urethane or just clear urethane over rust since these coatings are not air tight and are simply polyurethane designed for wood and concrete. Mastercoat Chassis Black Finish Coating is ideal over Mastercoat Permanent Rust Sealer on a chassis and on floorpans, especially with a paint brush, but not as a rust sealer alone. Mastercoat Chassis Black Finish Coating can seal rust and prevent from coming back it for a few years, but you'll get a much longer service life if you use Mastercoat Permanent Rust Sealer as a primer and Mastercoat Chassis Black Finish Coating as a topcoat.

Two coats of automotive epoxy primer, five coats of color, and two coats of clear will withstand 500 hours of salt spray. Two coats of Mastercoat Permanent Rust Sealer and one coat of AG111 Ultimate Finish Coating will do 14,000 hours of salt spray. Mastercoat Permanent Rust Sealer, if scratched, the rust will not go beyond the scratch. Epoxy primers are not air tight like Mastercoat Permanent Rust Sealer, so they are not very effective against rust, so rust will eat through it eventually.

NO. Mid-range relative humidity (50%) is perfect for these coatings. You don't want them to dry too quickly, which is what happens when you use them in high humidity. When the humidity is too high, you risk brush marks, bubbling, and poor adhesion.

Yes! Mastercoat Permanent Rust Sealer is solvent proof. Once it dries, you can use ANY kind of primer or ANY kind of paint over it. The sooner the other paints and primers go over Mastercoat Permanent Rust Sealer, the better, especially in hot and humid climates.

Mastercoat Permanent Rust Sealer was designed to go over marginally prepared surfaces. For instance, cleanup done with wire wheels and grinders. It works well over rust, but it works better over sandblasted surfaces. So the answer is not necessarily, but remember the old paint rule–the cleaner the better.

Yes. Over the years, both we and our customers have cars completely primed with Mastercoat Permanent Rust Sealer inside and out, especially in Florida where it is very humid. When painting over clean steel or galvanized steel, we recommend Mastercoat Metal Prep & Rust Remover to adequately etch the steel. This does a number of things: it changes the surface texture of the steel for better adhesion, kills the microscopic rust, and prevents future rusting. Mastercoat Metal Prep & Rust Remover is excellent for cleaning rust off parts and putting them away rather than coating them with a cheap primer.

Yes, by all means. Mastercoat Permanent Rust Sealer will seal a gas tank inside and out. Mastercoat Metal Prep & Rust Remover is used first to etch the inside of the tank. Mastercoat Permanent Rust Sealer SLIM is the optimal choice for coating the inside of gas tanks, since it sloshes easier for more even coverage.

Yes/Yes. Our Sprayable Slim formula is actually our ORIGINAL "original". We sold only this formula for decades before we introduced a version with thickener in it which made it good for brushing. Eventually this thicker formula ended up being called "original" due to its popularity and versatility, and then we later reintroduced the thinner sprayable version and named it "Slim". Customers have brushed on both versions for many years. The thicker "original" ordinarily requires a 10% reduction to spray, but it can indeed be sprayed on. The "Slim" version brushes on, but rolls on with a roller even better. The Slim formula is NOT pre-thinned. The only difference between the two is the original has thickener and the Slim does not. At the end of the day, you'll get the same long lasting results no matter which formula you use. The only difference in how you prefer to apply it.

It is best not to use them in temperatures under 40 degrees. This applies to all Mastercoat products.

Yes. You can coat the entire engine except for the exhaust manifolds and exhaust system. The other exception would be if your engine runs very hot, as Mastercoat Permanent Rust Sealer is air tight and may cause overheating.

Not much. Neither are "better" than the other. Both are the same exact formulas that will produce the same long lasting results, the only difference is the Original has thickener in it. The reason is for this is it makes it easier to brush. The Slim does not have the thickener, and is great for rolling on and spraying. Ordinarily you wouldn't have to reduce it, but if you do, start with 5% reduction with MCT101 or medium urethane reducer and use enough to get it through your gun.

350 degrees continuous. This applies to the rust sealers, black chassis coating, and AG111.

For the love of all that is good, DO NOT RECOAT WHILE THE LAST COAT IS TACKY. The previous coat MUST be dry to the touch before you recoat or else you run a very high risk of the coating bubbling and possibly even peeling off. And if that happens, YOU WILL HAVE TO SANDBLAST IT OFF AND START OVER. Seriously, do not do this.

Mastercoat Permanent Rust Sealer has been sold for many years under the title “Get Rid Of the Pits!”. Its high build characteristics and high solid content will usually take care of some major pitting, especially on hoods, roofs, and trunk lids. This is where you’ll really see the quality. You can apply multiple coats until those pits are filled in!

It is always better to use multiple thin to regular coats than thick coats. Moisture cured coatings are subject to blistering, cracking, and peeling if thick coats are used. This is why we recommend thin to regular coats every time over thick coats.

The Slim is not pre-thinned, no. The Original contains a thickener, which makes the consistency better for brushing. That is the only difference and the solid content of the two are the same, and produce the same exact long lasting results. So also no, you won't get more product if you buy the Original and thin it.

Slim rhymes with Jim. Please stop calling it Slime.

No, please do not mix any kind of hardener into the rust sealers. Moisture in the air is what cures it, so it is not needed.

Stir in an up and down fashion, NEVER IN A CIRCLE. When you stir moisture cured urethanes in a circle, it draws in moisture and drastically reduces the work time and shelf life of these products. Using a wood paint stick, use it to pick the solid content up from the bottom of the can and stir up and down to distribute the solid content throughout the body of the paint.

The shelf life of the rust sealers is one year unopened. These are approximate and usually last longer but are best used within this time. If they’re still liquid they should still be good.

The lowest would be about 30% and the highest would be 80%. We strongly recommend waiting for a day that is closer to 50% relative humidity, as this is ideal for working with these products. However we understand that some customers live in climates that are always very high humidity. In this case, you can apply the rust sealer in 80% humidity, but please understand this is still quite a bit of moisture for these materials and they may not behave as they would in 50% relative humidity. If the humidity is higher, do NOT apply thick coats--apply multiple thin coats, waiting until each coat is dry to the touch. Even if it is dry to the touch in 2 hours, it is best to still wait 3-3.5 hours before recoating. 

It depends on if you're using Metal Prep. If yes, use the Metal Prep to prep the surface first. If not, clean up the surface the best you can and then apply two coats of the rust sealer. This is followed by one coat of AG111 or Chassis Black Finish, or your own topcoat. 

DO NOT put these coatings on a shaker to mix them. Due to the material type, they could build up gases and become a hazard when you try to open them. The lid may blow off.

It is best to apply rust sealer after everything is welded. These rust sealers are not effective to weld through; you're better off using a weld through primer for that purpose.

Yes. Etch with Metal Prep first, and use an engine cleaning gun to spray the Slim rust sealer into the tank when the Metal Prep has dried thoroughly. Be careful not to spray too much/too heavy. You can slosh the rust sealer, but it’s important to stand the tank on end until every last excess drop of rust sealer has dripped back out or it could harm the baffles.

Generally yes, but only if your engine doesn’t run very hot. The permanent rust sealers and AG111 are rated to stand up to 350 degrees continuous heat exposure. The rust sealer creates an air tight seal, so if your engine runs overly hot, it can cause it to overheat.

Yes, the permanent rust sealers can be used on an oil pan and withstand the heat.

350 degrees continuous. So do not use these coatings on the exhaust manifold or anywhere it gets hotter than that.