Scuffing and cleaning a plastic bumper before adhesion promoter

Do You Need Adhesion Promoter to Paint a Plastic Bumper?

You need an adhesion promoter wherever paint goes directly onto bare plastic, such as a new unprimed bumper cover or a repair sanded through to raw plastic, unless your coating system says otherwise. With most plastics, scuffing alone doesn't give paint a mechanical grip. As a Sherwin-Williams product specialist puts it, "the exposed surface of that scratch is still plastic." If the bumper's primer or paint is cured, passes the solvent test below, and you don't sand through it, that coating is the foundation. Adhesion promoter then goes only where raw plastic shows.

How can I tell whether a bumper is raw or primed?

According to a 3M refinish trainer, "One of the first things to confirm is whether a plastic part is primed or raw." Two simple checks:

  • Solvent test: Hold a solvent-soaked rag on a hidden spot for 15 to 30 seconds. If the coating lifts, Sherwin-Williams says to remove it before refinishing. 3M describes a similar wipe test: the primer "is reversible if it's easily wiped away."
  • Sanding test: Sand a small hidden area. Look for primer sanding residue, or for a color change where you reach bare plastic.

Which plastics need adhesion promoter?

Promoter makers single out polyolefin plastics such as polypropylene (PP) and TPO:

  • 3M describes its 05907 promoter as made for "low surface energy plastics such as thermopolyolefin (TPO)."
  • SEM formulates its promoter "for olefin-based plastics."
  • Polyvance's refinishing guide calls for adhesion promoter on "PP/TPO substrates."

To identify the plastic, check the back of the part. Kurt Lammon of Urethane Supply Company says to look for symbols such as PP or PP+EPM. Urethane parts usually carry the symbol PUR. He adds, "If the bumper is black and waxy-looking on the backside, most likely it's a PP/TPO blend." I-CAR training notes that this identification code is molded into an unseen area of the part.

No single promoter covers every material. 3M says not to use 05907 on SMC or fiberglass-reinforced polyester repairs. Klean-Strip says its Bulldog VOC Compliant Adhesion Promoter should not be used on Lexan or polyethylene. Read the label for your product before you spray.

How do I prep and apply adhesion promoter on a raw bumper?

Clean thoroughly first. A raw part can carry mold release agents left over from manufacturing. According to Sherwin-Williams, some contaminants dissolve only in water-based cleaners and others only in solvent-based cleaners, so both kinds of cleaning are used. Clean the back side of the bumper as well. 3M adds that some paint data sheets call for a bake cycle on raw bumpers "to help sweat out the mold release agents."

After cleaning, the steps depend on the product:

Product Prep Application Before topcoat
Bulldog VOC Compliant Adhesion Promoter (TPO126) Abrasive Prep & Clean with a gray scuff pad, rinse, dry, tack One medium closed coat 5–10 minutes at 70°F or until tack-free; 24-hour open time
SEM Adhesion Promoter Scuff & Clean with a red scuff pad, rinse, then plastic cleaner One medium-wet coat 3–5 minutes; after 24 hours, scuff, tack, and reapply
Polyvance Bumper & Cladding Coat Plastic cleaner, wiped off; no scuffing or sanding One medium coat to hide Let dry completely; Polyvance says it can be topcoated at any time after application

These figures come from each manufacturer's data sheet. Bulldog is sold in more than one version, so follow the directions on the can you have.

Klean-Strip lists the TPO126 promoter as compatible with "all solvent-based urethane, epoxy, or acrylic lacquers, primers, sealers and basecoats." It can't be mixed with waterborne paint. However, the data sheet says it can be used in a waterborne system as long as it goes underneath the primer-sealer.

Do I also need flex additive?

Possibly. Flex additive is a separate product, and it goes into the coatings themselves when the paint system calls for it. PPG's data sheets for OMNI MTK single stage and MC270 clear both list MX193 Flexible Additive as an additive, at 10% of the ready-to-spray quart. 3M's refinish trainer describes plastic-part jobs using "a full coat of flexed sealer" and "a couple of coats of flexed clear."

Check the sheets for your sealer, color, and clear to see whether they include a flexible-parts procedure. If they do, follow their ratio.

What changes when I repair a damaged bumper?

On a repair, the promoter can go on twice. Polyvance's refinishing guide for PP/TPO:

  • Apply adhesion promoter before filler.
  • Reapply it over any raw plastic exposed during sanding, just before primer.

The guide's sanding steps:

  • Plastic weld: sand flush with 80 grit, then feather the surrounding paint with 180.
  • Filler: sand with 80 and 180.
  • Spot putty: sand with 220 and 320.

Our sandpaper grit chart covers these grits in more detail.

Plastic bumper painting checklist

  • Check whether the part is raw or primed with a hidden-spot solvent or sanding test.
  • Identify the plastic from the symbol on the back.
  • Clean the front and back to remove mold release and other contaminants.
  • Apply the adhesion promoter to every area of raw plastic, following its label for prep, coats, and timing.
  • Topcoat within the promoter's open time, using coatings it is compatible with.
  • Add flex additive to the sealer, color, or clear only where their data sheets call for it.

For choosing what goes on after the promoter, see our guide to epoxy primer, 2K primer, and sealer.

Plastic refinishing products at AutoColorOnline

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