Sand Scratch Swelling: Why Scratches Reappear Days After Painting
You did the preparation properly, laid down base and clear, the panel shines and the car goes back to the customer. A few days later the sanding marks start to show through. The usual cause is basecoat sprayed over primer that was still fresh and soft.
The solvent attack
When paint goes over acrylic primer that has not cured, the solvent in the paint attacks the primer chemically. It softens it again and makes it swell temporarily, because primer behaves like a sponge. Underneath there is usually body filler, another porous material that absorbs solvent.
What happens after delivery
On the surface everything looks perfect. Below, the primer and the paint are still fighting. The trapped solvent will leave over the following days. When it does, the primer settles and shrinks.
As it shrinks it pulls the clear down into its pores, and the lines of your sanding appear again. The substrate swelled, then sank back.
How to prevent it
Respect the primer's drying and curing times in the technical data sheet, especially at high film builds.
Do not sand or paint primer that is still soft. Force dry or IR cure it when the system allows.
Keep primer and filler film thickness within the manufacturer's limits.
High solids products help, but they do not cancel the chemistry. The problem starts when solvent meets primer that is not ready.
Frequently asked questions
What causes sand scratch swelling?
Solvent from the topcoat softens and swells primer that has not fully cured. When the trapped solvent escapes, the primer shrinks and the sanding marks show through.
How long should primer dry before painting?
Follow the technical data sheet for the film thickness you applied. Never sand or paint primer that is still soft.
Paint defects like this are a core topic in our seminars in Paiania. For restoration paint work, book a consultation.
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