Solvent Migration

Solvent migration is a effect where two materials are in contact and solvents in one migrate to the other. In much the same way as if you were to put a wet towel on a dry towel, the water will move from wet to dry, if you have two materials which are both permeable by solvent or plasticizers next to each other, if one is more 'wet' than the other, the solvents will slowly migrate.

An example of this, when self adhesive stickers are placed on a vehicle, solvents from the adhesive and the vinyl can migrate into the paintwork. These areas might cause the paintwork to swell, be softer, or have a sheen.

Another example occurs when a car is in storage for a long period of time and plastic items are placed on the car. This might be a plastic tarp, plastic bags, plastic tool boxes or even synthetic carpet or fabric. Solvents, including any dyes or pigments, may migrate into the paint and cause stains.

What it means

Solvent migration refers to what happens when solvents trapped in primers, fillers, basecoats or even older paint layers gradually move into the fresh topcoat or clearcoat. If earlier coats were applied too heavily or not given enough flash-off time, they can still contain solvent when the next layer goes on. As everything continues to cure, that solvent slowly moves upwards and out, disturbing the top surface and causing loss of gloss, shrinkage lines, edge-mapping around repairs and, in some cases, staining or discolouration.

Why it matters

• Finish can “die back” after delivery: A car that looks glossy when it leaves the booth can lose gloss and appear dull or uneven days or weeks later as solvent migration takes its toll.
• Repaired areas can “print through”: Edges of filler, primer patches and previous repairs can start to show as faint rings or maps in the topcoat once underlying layers continue to move and shrink.
• Can cause long-term instability: Where strong colours, stains or old repairs are involved, migrating solvents can carry pigments and contaminants, leading to patchiness or ghosting that is hard to polish away.
• Usually means rework: Mild issues might be improved with flat and polish, but more serious solvent migration often requires sanding back, correct curing and repainting the affected areas.

Where you’ll see it

You’ll see solvent migration mentioned in bodyshop technical bulletins, paint manufacturer datasheets and rectification notes, often in connection with die back, loss of gloss, shrinkage around repairs or edge-mapping. It is most common on larger refinished panels, older cars with several paint histories, and areas where heavy filler or primer work has been carried out before topcoating.

Context

Solvent migration sits within the family of solvent-related paint defects, alongside solvent pop, die back and sinkage. Whereas solvent pop shows as pinholes from rapid gassing, solvent migration is more of a slow, creeping change in the film as trapped solvents move and evaporate over time. Prevention relies on following the paint system correctly – sensible film builds, correct thinners, proper flash-off and bake times, and care when refinishing over older or unknown paint systems. Good preparation and patience are usually cheaper than trying to fix defects that appear after the car has left the shop.

Common mistakes

• Loading on heavy coats of primer or clear and not allowing proper flash or cure time before the next stage, trapping large amounts of solvent in the film.
• Refinishing over unknown or poorly cured old repairs without testing or sealing them properly, then seeing edges and maps reappear weeks later.
• Trying to fix serious solvent migration with a quick machine polish, which may briefly improve gloss but does nothing to stabilise the underlying problem.
• Ignoring paint system datasheets on film build, thinner choice and drying cycles, leading to repeated die back and shrinkage on the same panels.

Written by Danny Argent. Last updated 19/11/2025 15:44