Bare metal is the starting line — here's how to get there clean
AIS builds the exact belt for your prep work — any grit, any size, no minimums.
We don't need to lecture you on just how impactful to the build process the metal you're working with is. You can learn a lot of what sort of work you're in for just by the way the metal looks long before the fabrication work begins. Choose your grab bag of starting points: rust that wasn't fully removed, mill scale that got sanded over rather than taken off, old finish that was stripped in patches - all of which don't disappear once the work starts. You can expect these difficult problems to show up later, usually at the worst possible moment, either under a weld or under a coat of paint.
While getting to clean and bare metal isn't complicated, it does require matching the right abrasive to what's actually on the surface. Rust, mill scale, and old coatings each respond differently to abrasive work and treating them all the same way is one of the more reliable ways to either remove too much material - or not enough.
Rust - Surface vs. Deep
When rust is the issue at large, the first step is to establish just how deep it goes. Surface rust and it's telltale orange-brown oxidation pattern that forms on steel exposed to air and moisture sits at that surface level and can be removed cleanly with abrasives without significant material loss. Pitting and scale rust up the difficulty a bit just based on their nature. Pitting means that the oxidation has eaten into the metal surface, leaving a cratered profile that abrasives will clean - but not flatten. Scale and it's thick, flaking layers of iron oxide, needs to be addressed before any fine finishing work - because these scales will load up on even the newest belt and set you back in time, and cost for the wasted materials.
For surface rust removal, Aluminum Oxide in the 36-60 grit range is the practical starting point on most mild steel. It removes the oxidation efficiently without cutting aggressively into sound metal underneath. On heavier surface rust where you're dealing with more material to move, stepping up to Zirconia-Alumina at the same grit range will cut faster, and hold up better through the pass.
Don't confuse clean with flat. Abrasive work removes rust but it doesn't fill pits. A surface that looks clean after prep may still have a pitted profile that will show through primer and topcoat on panels where appearance matters. If you're doing cosmetic work on a surface with pitting, the abrasive gets you to clean - body filler or skim coat gets you to flat. Knowing which problem you're solving before you start saves a lot of backtracking
Mill Scale - The One that Looks Fine and Isn't
Mill scale, the dark, bluish-grey oxide layer that forms on hot-rolled still during the rolling process. It's the triple whammy of being harder than the steel underneath it, chemically different from rust and does not play well with welds or coatings. Welding over mill scale produces porosity and weak fusion zones. Coating over it means the coating is bonding to the scale rather than the steel - and when the scale eventually releases, the coating goes with it.
Due to all these potential issues, mill scale requires a different abrasive approach compared to rust. Because it's harder than the base metal, you'll need a different grit than what you'd use for rust removal. For instance, a 60-grit Aluminum Oxide belt that cuts surface rust efficiently will skate over mill scale without making much progress. What you'll need is a coarser grit - 24-36 - with a mineral that has enough hardness and toughness to bite into the scale. That leaves the standard call: Zirconia-Alumina where the toughness compared to that of Aluminum-Oxide makes the practical difference.
Stripping Old Coatings - Loading is the Real Problem
Old paint, primer, and coating removal is the application where abrasive loading kills more belts than wear does. Paint and primer are soft and resinous - they fill the space between abrasive grains fast, turning a sharp cutting surface into a smooth, ineffective one. The result is heat, slow cut rate, and a belt that looks like it has plenty of life left but isn't doing any real work.
The easiest fix for this issue is going to be by using open-coat abrasives, these are specifically designed to give the swarf somewhere to go rather than packing on the abrasive surface. Aluminum-Oxide open coat at 36-60 grit handles the majority of old coating removal. The more difficult, layered, or particularly tenacious coatings - epoxy primers, industrial coatings, thick undercoating - a coarser starting grit or a Zirconia open coat will cut this more aggressively. While we don't have any open coat belts available on AIS.com, you can still submit a Quick Quote Form and we will work with our vendors to see what we can get for you!
One last thing worth checking before you start: if the vehicle or part is old enough that lead-based paint is a possibility to be mindful of the fine particulate that standard abrasive stripping generates. It's not a reason to avoid the stripping process - with proper respiratory protection and disposal practices you can stay a step ahead - but it's a reason to know what you're getting into before you start.
How Clean is Clean Enough?
There's no one answer that question, instead it's dependent on what comes next. For welding, clean bare metal means no rust, no scale, no coating within at least an inch of the weld zone - ideally more. Contamination in the weld zone doesn't just cause cosmetic issues; it creates porosity, inclusions, and heat-affected zone problems that compromise the weld structurally. For coating, clean means no oxidation and a surface profile that gives the primer proper purchase. For the desired mechanical bite, that's typically a 60-80 grit scratch pattern, though for etching and epoxy primers you'll find more specialized surface profile requirements so be sure to check that product spec!
