Applying a Primer Surfacer
The goal when applying a primer surface to a panel is to provide holdout and protection over your substrate. Primer surfacers provide some fill to level out feathering and correct minor flaws on your panel. Any dent repairs involving body fillers should be finished in P180-P320 grit paper before starting a priming operation.
Clean your Panel
Before you begin sanding, thoroughly clean your panel using soap and water, followed by a wax and silicone remover. Also inspect your panel for any other damaged areas that may need repairing before starting to prime.
The purpose for paint feathering is to create a gradual taper and prevent a hard edge. This will make it easier to level and avoid the appearance of any rings caused from sinking later as the paint cures. Feathering can be done with a sander or by hand, using a block can help give you a perfect taper. Use sandpaper ranging from P180-P320 and attempt to separate the paint layers about 1/2” between layers (colors).
The purpose of backsanding is to allow your primer to always land on a sanded surface. Since you will want to prime over your feathering you will be required to bring your primer a bit further and it needs to be applied over a sanded or “backsanded” surface. Backsanding is done using P320-P400 grit paper, either by hand or machine. You will want to backsand about 3-4” beyond your feathering. On small spot repairs it may be necessary to reduce the length of your backsanding for the purpose of saving blend space.
- Scuff up any edges with a red scuff pad where sandpaper is not accessible.
- Clean your panel once again with an appropriate final wash cleaner.
- Mask off your panel; leave about 6” past your backsanded exposed.
Selecting a Primer
Your primer choice is dependent on the substrate you are priming over. For bare metal you may be required to use an etch or epoxy primer before applying your urethane (2k) primer surfacer. There are direct to metal urethane primers for one step application, contact your paint supplier for primer options. Raw plastics will require adhesion promoters and sometimes flex additives in the primer. Each paint manufacturer differs slightly so you will need to chose your products accordingly.
Applying your primer
If you are applying an epoxy, etch or plastic primer (adhesion promoter) you will likely be applying them lighter then your urethane primer surface, consult the technical data sheet for specific information on each product. They are generally available on your paint manufacturers website.
Next before applying your urethane primer surfacer , consult the technical data sheet to find out the recommended application on your primer. It will provide information such as equipment specifications, tip size, spraying pressure, number of coats and flash time between coats. Follow the information provide. Apply your primer wet similarly to clear coat, the smoother your primer goes on the less sanding you will have to do. Use slower hardeners on bigger jobs and hotter climates to help get your primer as flat as possible.
- Spraying over un-sanded areas. This is generally because the surface wasn’t back sanded far enough; give yourself as much room as your can. If you see shiny edges when you begin sanding that is the likely cause. It can lead to the edge lifting if not repaired properly.
- Feathering out instead of in. When using a DA sander you can achieve a better transition by sanding from the outside in rather then inside out. Sanding from the inside out can cause a tear instead of a transition that is only observable under a microscope. This can lead to sinking and rings after the paint cures.
- Priming to heavy. This is generally the result of burying a defect in primer that can lead to dieback, cracking and other problems. Avoid over priming or priming to heavy.
- Prime from the outside in. The proper way to prime is to bring each coat inward and not to prime over the previous coats overspray. This can also lead to sinking after the paint cures.
- Priming to an edge. Never prime all the way to a tape line. If you create a hard-edged primer line there is a very good chance that line will appear in the finish once painted. Always give yourself lots of room and remove overspray with fine paper of paint thinner if necessary.