RULES FOR SANDBLASTING AUTOMOTIVE PANELS WITHOUT CAUSING DAMAGE OR WARP
Hoods, deck lids and top panels require special instruction and 60/120 grit black magnum to prevent damage to these panels. As a general rule, blasting pressures should be maintained below 35 psi (pressure pot type systems) for all automotive sheet metal applications including the exterior surfaces of hoods, deck lids and top panels. The undersideof these panels must be blasted with extreme attention to the details that follow or damage will result.
All framework and support structure not directly part of the top surface of these hoods, deck lids, or top panels may be blasted directly at no more than 35 psi. Care should be taken not to blast open areas on the underside of hoods, deck lids, or top panels in between the support structures until the following rule of thumb test is applied. A piece of thin lexan is often useful to shield areas that cannot be blasted around this support structure framework.
The underside of a hood, trunk lid, or top panel may not be blasted if it can be depressed inward using thumb pressure. Using only your thumb apply pressure from the top side of these panels inward. If the sheet metal can be "oil-canned" (pushed) inward then no abrasive blasting or other metal removing process (this include d/a, grinder, etc.) may be used directly on the underside of these areas if it will result in metal being removed. Do not remove any metal from these areas by any means or else damage will result. The panel will begin to sink inward toward the concave side as far as the rule of thumb will allow it to go. The more metal is removed from the wrong area the further the panel will sink inward. Sheet metal always "grows" towards the side being blasted. This is not warp. Warp is totally different and the result of excessive pressure and heat along with high coefficients of peening force resulting in a washboard pattern.
Any area not showing inward movement passes the rule of thumb and can be blasted on the underside without incident. Generally this includes the center line bead, side strength beads and most depression inlets around air scoops, heavily rounded side corners, nose pieces, edges and all inner frame work or double inner panels ( most common on deck lids ) that are not part of the underside of thetop surface of the panel.
The entire top side of hoods, deck lids and top panels may be blasted without incident as long as the process is completed evenly and with the proper pressure controls (35 psi max.) and correct media(s).
Never partially blast the top side of these panels and then stop part way thru and decide to use an alternative method of paint removal.
Always start blasting with the front edge of the hood and work from front to back and not side to side. This will have little effect when everything is completed but will prevent panic attacks and second guessing during the process which may cause you to stop and try something different that will cause permanent damage.
Always strip the top side before progressing to the underside and do not deviate from these instructions.
Never start with a hood.
Never start with a deck lid.
Never start with a top panel.
Always start with anything but a hood, deck lid or top panel if you are inexperienced at blasting sheet metals. This will help you learn to maintain a correct blasting distance ( approximately. 9 inches) and provide ample opportunity to work out inconsistencies in blasting equipment and pressure controls on less delicate sheet metal components not as likely to be damaged with abuse or mistakes.
Always strip one layer at a time and work in 1 square foot areas. This helps keep heating of the metal to a minimum and provides the fastest stripping speeds on soft paints such as lacquers.
Never strip more than 1 or 2 layers of paint at one time.
Never dwell in one location for more than a few seconds.
Never holding the nozzle too close to the part being blasted.
Always maintain a proper distance in relation to the part of approximately. 9 inches.
Do not start in the middle of a panel and then move around to different locations or strip all outside edges and then work towards the center in a spiral design. This will result in unnecessary panic attacks. Remember to finish that which you start.DO NOT CHANGE SETTINGS! DO NOT DEVIATE FROM THESE INSTRUCTIONS! Everything is going to be alright.
Do not exceed 35 psi or use a round particle (this includes glass beads, sand, star blast, cinter balls, ceramic shot, steel shot, etc.).
Always use an angular particle not to exceed a 60 grit in size for hoods deck lids or top panels. All other panels may be stripped with a 30/60 grit black magnum or other angular grit ( fenders, doors, valence panels, wiper grills, quarter panels, etc.) while not exceeding strict pressure controls of 35 psi or less. Any angular abrasive having a maximum weight density of 150 pounds or less per cubic foot is acceptable. This would include aluminum oxide, black magnum, silicon carbide, crushed glass, novaculite, etc. If using walnut shell, plastic, or other light media having a bulk density of less than 60 pounds per cubic foot, do not exceed a 12/20 grit blasting particle size and maintain pressure settings at or below 35 psi.
Never start blasting unless you have clearly understood these rules and fully intend to complete each panel that you begin work on without exception.
Always ask for clarification if you have not completely understood these rules and have confidence in your ability to complete what you start. We stand behind these blasting techniques and will make ourselves available to offer any additional information or clarification about these rules and / or refute other bad advice or experiences you may have had with regard to media blasting antique vehicles. These rules have been tried over thirteen years and are proven to be truth. As with all other sources of knowledge, it will only help you if it is applied properly with understanding. Good luck. Have a blast and God bless.
Don E. Schmidtke and company
DIFFERENT GRADES OF QUALITY IN SANDBLASTING
ALL FOUR GRADES ON ONE PLATE
Starting with the top right picture and rotating counter clockwise, the four pictures represent grades D, C, B, and A. In the blasting industry, price sometimes becomes the single largest factor in weather a company wins a bid or loses a job to the competition. It is therefore understandable that most sandblasting companies price their work based on grades D, and C. These grades of quality leave much to be desired, however. Commonly referred to as a commercial blast, these grades don't come close to the quality standards of a white metal blast. In the end there is no way to tell what's under your paint, primer or powdercoating until the stuff starts peeling, chipping, or rusting straight through the coating. Then of coarse, it's to late to worry about.
It is an unrefuteable fact that the better the grade of blasting you obtain, the longer the coating you apply is going to last. It's all about adhesion, or lack there of. A shiney surface will provide less surface area at the substrate level than a dull or flat matte finish. By abrading the surface of a substrate with blasting, one creates a greater amount of surface area and the surface becomes more and more dull. During sandblasting operations, metal removal begins once the paint or other coating is removed. This is the process at work in abrasive blasting with silicon carbide, aluminum oxide, silica sand, coal slag, copper slag, barshot, steel grit, crushed glass, garnet, and many other angular grits used in sandblasting. All the above can provide a white metal grade A finish on metals. Glass beads, walnut shell, corn cob grit, plastic medias, and numberous other mildly abrasive and nonabrasive medias will remove coatings but will not necessarily be capable of obtaining white metal finishes. They simply are not designed to remove metal. They all have their proper use and we use many of them for specialized surface preparations. However, a white metal finish will produce better adhesion between substrate and coating and thus, increase longevity.
At our facility, we blast everything to a white metal or near white metal surface unless a customer demands a lower quality. Obviously time and price will be commensurate with the quality of any job. The typical difference between a commercial and white metal blast can be as much as double the cost, especially when encountering mill scale as pictured here or other heavy pitted rust. The choice is yours to make, but if you don't specifically demand white metal you're probably receiving a commercial blast and that can be the most costly job of all.