When working on spot repairs or overall paint jobs the painter must allow for proper flash and cure times between coats of the same product and primers and top coats.

Sometimes especially when performing spot repair jobs in a busy shop these wait times are ignored or pushed however this can result in call backs when solvents in the paint need to escape and either cosmetically or physically damage the job.

Each paint manufacturer will supply a Technical Data Sheet that provides exact information for the product you are using and how it may be used with other products.

For this howto we will give general times that you should wait before continuing on with your work.

Lacquer Primer Surfacer

Acrylic Lacquer Primer is not normally used today but you may still find it. If you are using this product the wait times for flashing is 15 minutes flash time between coats and 30 minutes before sanding. Since this product drys to the touch quickly you may decide to begin sanding earlier then the specified wait time but even with UV dryers it is recommended that you wait a full 30 minutes before you begin. After you have completed sanding and cleaned the surface you can immediately apply a primer sealer or your top coat.

Primer Sealer

When using Acrylic Lacquer Primer Surfacer the surface can be left without a top coat for approximately 3 days after sanding but prior to top coat application however because so much dirt and oil could find its way to the surface it is important to seal the surface prior to applying your base coat. Not doing so is really up to you but it is best practice. If you expect to leave the vehicle in primer longer then three days you should seal the primer surfacer to inhibit air borne water from causing rust. Then you can scuff the primer sealer with 400-600 grit sand paper prior to top coat application.

Urethane Primer

Urethane Primers take slightly longer to flash and cure. Although the same 15 minute flash time is recommended between coats the maximum number of coats is two for this application. If you are building more then 3 mils of surface to correct problems in your substrate such as pitting or unlevel surfaces you must extend your wait time between coats. A full 3 hours is recommended before sanding. Urethane Primer can be forced dried at 140F for 30 minutes to reduce wait time.

Urethane Primer does not require a Sealer for extended storage times. If you do not expect to paint the vehicle with top coat for an extended period of time then you should wait to sand the surface or spot prime any areas that you break through during sanding.

Epoxy Primer

Epoxy Primer is often used by restoration shops or home builders. It is said to have extended rust protection properties but I would personally never use it on the exterior or any exposed surface. It is better suited for underbodies and suspension parts in my opinion.

If you are using this product you should refer to your TDS for specific information about top coating however this product normally requires double the flash time of 10-20 minutes between coats and 90 minutes to cure before sanding.

If you expect to apply your top coat without sanding you must wait a minimum of 30 minutes to a maximum of 24 hours.

I would actually recommend 3 hrs minimum before top coat and you must sand the surface if you wait longer then 24hrs

Final Note

Primers all have different properties and require different handling because of the chemicals used in their formulation.

Primer Surfacers really should have an extended cure time because you will be sanding them. I personally like to give the vehicle overnight before I begin sanding but if you are completing a spot repair you should adhere to the minimum curing times then complete your top coat within the same day.

Primer Sealers can be used in conjunction with primer surfacers to provide a moderate amount of protection from surface rust due to humidity in the air. Unfortunately unless you live in Nevada in the Desert you can not wait forever to apply a top coat or you will need to sand to metal or to your previous top coat and start again. But a week or two in primer sealer is fine if you scuff the surface prior to top coating.

There are a variety of different primers that we did not cover because they are for specialty applications. We will cover them in other HowTos but if you are working with any product refer to the Manufacturer’s Technical Data Sheets before you begin.