We’ve all seen the big custom builders on TV burying their apparently perfect bodywork in a skim coat of filler. And then the rest of the episode seems to show them sanding off 90% of what they just laid down. Like us, you’ve probably wondered why even bother if most of the material ends up on the floor anyway. It’s an age-old question that can spark heated debates for and against the practice – with no right answer.
To be clear, there’s a difference between skim coating a well-prepared body and simply burying one in filler as a shortcut. We’ve all seen shoddy work from shady shops that have applied a river of “mud” to avoid doing proper bodywork. Skim coating – sometimes called a “blocking coat” or “float coat” – is not that at all. Instead, it’s the process of applying a thin layer of filler to all surfaces as a leveling coat for block sanding.
A skim coat is applied after all metal-working is completed, epoxy primer has been sprayed, and any spot filling has been done. The material itself is typically either a urethane or polyester composition. Depending on the product, it might be sprayed on (sprayable primer/surfacer) or spread (body filler or glaze). These products are then block sanded to create a uniform surface on which to apply paint.
But while skim coating itself is an acceptable professional practice, it’s not always necessary for every job. In fact, car manufacturers don’t even do this on the assembly line, and for good reason. It adds cost in both materials and labor time to improve a finish that might be unnoticeable to most observers. So, when is it right, and when is it just wasted time and money?
The Case for Skim Coating
Because of the added time and expense of laying down and blocking out an additional layer of filler, most of the time this process is reserved for high-budget custom builds and serious restorations. But even a more modest project can benefit from at least partial skim coating where previous damage has been repaired. Here are the situations where experienced restorers or body techs often recommend a full skim coat before block sanding:
Show-Car or Concours Restorations
When owners are paying six figures or more for perfection, panels must be laser-straight and ripple-free under harsh lighting. Even new factory panels have minor waves; skim coating and long-blocking removes them. A uniform filler layer makes the surface behave consistently under the sanding block, ensuring ultra-flat reflections in final paint.
Heavily Worked Panels
Localized filler patches will sand at different rates and leave “highs and lows.” A full skim on large sections that have been hammered, welded, or patched creates a uniform sanding surface, blending the repair area with surrounding metal.
Panels with Widespread Minor Damage
Hail-damaged hoods and roofs, panels with numerous dings, or overall “wavy” sheet metal can be time-consuming to spot-fill. Chasing dozens of individual low spots can be less efficient that just working the whole surface. Skim coating lets you address all imperfections at once, then block the surface smooth.
High-Gloss or Dark Paint Finishes
Dark colors and high-build clears act like a mirror. A skim coat reduces the risk of subtle surface flaws showing after paint, preventing “read-through” or distortion that highlights imperfections.
Panel Replacement / Aftermarket Sheet Metal
We all know reproduction panels often don’t match OE tolerances. A skim coat can help refine contours, edges, and transitions to factory or adjacent panels.
When to Skip the Skim Coat
As already mentioned, skimming adds time and material costs to the bodywork process. And not every project requires the added effort. Here are situations where, regardless of budget, it probably just doesn’t make sense to skim coat.
Straight, Undamaged OEM Panels
When installing factory panels with minimal wear, no dents, and only light surface imperfections. Here, applying a skim coat adds unnecessary labor, cost, and material thickness. Spot putty, glaze, or primer-surfacer is more efficient.
Collision Repairs Following OEM Guidelines
When manufacturers emphasize localized, minimal filler use and restoring shape through metal work first. A skim coat can exceed recommended filler thickness or coverage as outlined in collision industry guides like I-CAR and 3M/PPG.
Light Refinishing or Partial Repaints
When blending panels or refreshing existing paint with minor prep. Reason: Full skim would unnecessarily disturb good substrate. Instead, technicians spot-fill and then use high-build primers and block sanding.
Weight- or Thickness-Sensitive Restorations
When restoring classic or performance vehicles where excess filler could add weight, risk cracking, or hide metal details (e.g., factory stampings, seams). Skim coating creates uniform thickness but can obscure originality and, if applied too thick, increase the chance of future failure.
Panels Prone to Heat or Flexing
On hoods, roofs, or thin sheet metal areas subject to vibration, engine heat, or sun exposure. Here. large-area skim coats are more susceptible to shrinkage or cracking over time if the panel flexes or the filler layer is too broad.
Conclusion
In short, full skim coats make sense when absolute flatness, consistency, and efficiency across large areas outweigh the cost of extra filler and labor. They’re especially common in restoration, custom, and show work – less so in everyday collision repair, where manufacturer guidelines limit filler use.
Use a full skim only when it solves a real problem (waviness, widespread damage, show-car requirements). For everyday collision or light restoration work, metal finishing, localized filler, and high-build primer usually deliver professional results without unnecessary filler coverage.
Jim, the longevity of the paint job shouldn’t be affected by the process of skim coating. It’s critical that any body filler primed and sealed before base coat application. This ensures consistent adhesion and uniform color. The key, as always, is the quality of the work being performed.
Bryan, The question of longevity of paint job was not addressed. Still looking for that answer. One might have to assess older restorations to find the answer. Still wondering. Thanks for the in depth response.
James, thanks for the question. I ran this past our product development team who collectively has years of experience with paint and bodywork products. Here’s the feedback I got:
While skim coating the entire vehicle may “look” like a lot of filler during application, once blocked the filler will still only lay in the low areas. If you could utilize guide coat with a magnification lens you could “selectively apply filler to all the minute low areas”- but this is not feasible so the process of skim coating the panels achieves the same outcome, by filling every low no matter how in perceivable to the eye.
Once blocked you will still only have filler in the low spots. The process of skim coating is then followed up with high build primer, this will also cause detection with a digital or magnetic thickness meter which this customers management may be confusing with the skim coat process. Any quality polyester filler when mixed correctly and applied over properly prepped panels and 80 grit sanding scratch patterns will adhere without issues up to 0.125” as most manufacturers recommend.
Customer should first use thickness gauge on area not prone to filler such as A- pillar to first get a base line coating thickness, then proceed to lower portion of door, rockers and front and rear of all wheels these are the area’s most prone to damage repair and “heavy” filler work.
I have a question. I am an agent for a company called Auto Appraisal Group. We use a paint meter of course when inspecting vehicles.. Skim coated cars often we get paint meter readings that the office calls filler and therefore rates the vehicle lower. My question is, Does skim coating affect the longevity of the paint job versus no skim coat?. I have disagreed with my office on rating vehicles that have a lot of skim coat. Looking for the answer. Good article.