Questions for Larry

November 1, 2009 | By Larry Lyles

Readers Ask This Restoration Pro About Sanding, Nozzles, Compressor Ratings, Body Shells, Polishing & More.

Why Are Some Compressors Rated “scfm”?

I have a few questions concerning the compressor article in the July issue.

First off, my compressor is rated with scfm instead of cfm. What’s the difference in the terminology?

Second, although you said the ratio of change between cfm and psi is exponential, is there a rule of thumb that could be applied to various situations?

For example, if you reduce the psi by 10 does the cfm go up by 1.5?

Jeff Britt Via e-mail

The difference between an scfm rating and a cfm rating is basically ideal conditions compared to real world conditions. Most compressor manufacturers like to use the scfm rating in order to standardize their units, which is what the “s” stands for: Standard cubic feet per minute.

So what’s standard? The air temperature must be 68° F. The relative humidity must be at 36%. The air must be measured at sea level.

Working outside of these ideal conditions will change the actual cfm output of a given compressor, but not enough to make any difference.

I’m not aware of a “rule of thumb” for determining the ratio between cfm and psi output of any compressor.

There is a huge, long formula, one that even I don’t understand, that can be used to calculate the ratio between cfm and psi, but it requires the input of many different variables, including which tool is being used.

The bad news here is that if you adjust the inlet air pressure on the gun you change that ratio. So for our use the formula is pointless.

I can add that if your compressor is rated to produce 12 cfm at 40 psi then certainly you will have more available cfm at 30 psi and even more at 25 psi.

Tell Me About Nozzle Sizes and Paint Cups

I’m looking at the Eastwood Concours spray gun #11605 and would like information on what nozzle tip sizes are best for a given application and why the gun comes with a plastic cup and a stainless steel cup. Does each one have an advantage? I am a novice at this and any help is appreciated.

Jay Roth Via e-mail

I’ll answer the nozzle question first. The Concours gun comes with three spray nozzles, a 1.2mm, 1.4mm, and a 1.8mm. In most applications use the 1.2mm tip to spray sealers, base color coats and clear coats. Use the 1.4mm tip to spray urethane primers. Use the 1.8mm tip to spray heavy body products such as epoxy primers. This is just a rough guideline. Always read, understand and follow the spray nozzle recommendations printed on the data sheets that are supplied by your paint dealer. Those sheets will specifically state which nozzle to use when spraying the product.

As for which cup is best, that’s a painter’s preference. I prefer the stainless steel cups but won’t reject a spray gun just because it comes with a plastic cup. The weight difference is negligible and cleanup is the same.

I don’t like to use this forum to hawk our products, but since you mentioned being a novice you might want to check out our “Basic Painting Techniques” DVD as well as our “Curing Paint Problems” DVD. You will find a lot of great information in both of them. Visit lpl bodyworks.- com for more information.

Rotisseries Leave an Unfinished Spot

I have my Camaro on a rotisserie and I will be applying the epoxy primer soon. Looking at the car, I’m wondering what to do around the mounts where the rotisserie attaches to the body. There is a small square area that I will not be able to paint. What do I do about that?

David Jenkins Bayville, New Jersey

Good question. To start, I always mask off the rotisserie mounting arms to keep paint from getting on them. Once I have the epoxy, color, clear or whatever I’m applying to the underside completed and have unbolted the rotisserie from the body, I mask off the area that didn’t receive any paint, sand it, apply epoxy,then refinish the area with whatever I used to refinish the rest of the underside.

Once the masking tape is removed, that little square blends into the rest of the finish. I may know that little “patch” is under there, but once the body is back on the frame it is well hidden and definitely well taken care of as far as rust prevention is concerned.

Is Your Lizard Skin Inside Out?

I am considering the Lizard Skin sound deadening and ceramic insulation products you used on Project ’46 and I would like to know how the products are holding up after having been on the car for more than a year now.

I would also like to know why you used the ceramic product on the underside and the sound deadener on the inside. When I contacted Lizard Skin they recommended just the opposite, the ceramic on the inside and the sound deadener on the underside.

Dave Coleman Via e-mail

The Lizard Skin products are holding up very well; no peeling, no chipping and the car rides very quietly. I am extremely happy with the results and won’t hesitate to use these products in the future.

As to why I elected to use the ceramic product on the underside, I had a long chat with the folks at Lizard Skin concerning my intentions with the products. I wanted a bulletproof coating on the underside that would stop rocks kicked up from the tires from denting the fenders and to have a coating that I could paint. The ceramic product filled that need perfectly.

I could have stayed with the ceramic coating on the inside of the car but I wanted the opportunity to try the sound deadening product. What I ended up with is a car that is quiet to drive despite the loud mufflers.

For more on Lizard Skin products, visit

Getting Good Coverage on Door Edges

I am working on a ’65 Fairlane and at this point I am ready to paint the doors off of the car. By the way, I constructed your PVC spray booth to do my spraying. (See the December 2006 issue.)

My questions are:

How can I ensure good coverage on the outside edges of the doors? What angle do you spray from?

Tim Smathers Via e-mail

I treat all edges just like they were panels. I hold the spray gun 90 degrees from the surface.

That means if the door is laying flat on a fold-out work bench, I start by aiming the gun straight up to get the underside of the edge, move the gun to face the thin outside edge, and finally rotate the gun up to spray down onto the edge.

The edges are always sprayed first, followed by the flat surfaces. That ensures good coverage everywhere.

Don’t Sand That Paint Just Yet…

I just had my 1957 Chevy painted with DuPont Centari acrylic enamel.

When the car is parked in the sunlight, the finish looks like glass. But when I move the car into the cool shop, the finish has orange peel.

Will the techniques used in the 3M article in the May issue (“Produce a Mirror Finish”) work on enamel to sand out the orange peel?

Les Bleil Via e-mail

The techniques will work just fine as long as the finish is NOT a metallic color and the painter applied a minimum of three coats. However, I wouldn’t get in a hurry to sand the finish.

From your letter it is obvious the enamel hasn’t had time to completely cure out. What is happening is that when you park the car out in the sunlight, the finish warms and it begins to flow out and flatten. When you move the car inside, the finish cools down and softens a little; thus the orange peel effect.

I’d give the finish at least a couple of months to cure and then sand and polish it. I think you will like the result.

Tips For Mounting a Stinger Hood Scoop

I want to install a 1967 Stinger hood scoop on my 1976 Sting Ray. I have purchased the scoop but have a few questions about how to mount it. First, the L-88 hood that is now on the car will need more clearance for the 2-4 set-up I plan to use. That means I need 4 5 ⁄8 inches more clearance than the stock hood provides. Can I use Styrofoam and cut it to help mold the scoop to the hood? If so,should I cut the stock hood before or after I mold the scoop to the hood?

Larry Emmons Wyoming, Michigan

I would begin by positioning the scoop on the stock hood where it looks and works best.

Then outline the scoop using masking tape. Remove the scoop and make your cut an inch inboard of the outline.

Mount the hood back on the car and use the Styrofoam to construct the mold to raise the scoop enough to gain the necessary clearance for the air breather. Use hot glue to hold the foam to the fiberglass.

As you mentioned in your letter, getting the Stinger part of the scoop to “look right” will take some trial and error fitting. I would use thin cardboard, much the same as I used in Project ’46 to build up the headliner under structure, to mold the stinger into the stock hood.

Once you have the scoop positioned the way you want it, remove the hood from the car, flip it over and glass the scoop to the hood on the inside. That will get everything stabilized so that you can flip the hood back over, sand off the Styrofoam and cardboard and do the glass work on the outside.

All of this will take some effort and time but isn’t that difficult in the long run , click on the “Projects” button and check out the silver Corvette. I did a similar reconstruction to that hood to gain clearance for the twin turbos added to the engine.

By the way, use a poly-based fiberglass resin, not SMC resin, as I’m sure the scoop will not be constructed from SMC.

Tell Me About Aftermarket Mustang Body Shells

Are you familiar with the aftermarket Mustang body shells that are being sold today? If so, can you tell me about your experience?

John Sutter Piscataway, New Jersey

I’ve heard about them, but I have yet to put a hand on one. That makes my experience building one of these cars non-existent. All I can offer is hearsay evidence from a good friend in the restoration business who has actually built one. I’ll leave out the expletives my friend offered and edit his experience down to actual hammer swinging. I quote: “Nothing fit. We had to modify everything. We spent twice as much time and money as we would have spent locating, retrieving and building a rust bucket from Ford.”

Life Expectancy of a Sanding Disk

You didn’t mention how many sanding disks you went through in the May article on getting a mirror finish. Does it depend upon the quality of the machine? I use a Craftsman DA and it works pretty well for me.

Ken Crystal Via e-mail

I tend to go through one 1500-grit sanding disk per panel. The disk would probably last through two panels, but not much more than that. The 3000-grit disks last a lot longer. I tend to go through three to four disks for a medium-sized car. I’m talking wet sanding here, not dry. I don’t think they will last long if used dry.

Comparing Some Older and New Polishing Products

Is there much difference between the new 3M polishing products you used in the May article on getting a mirror finish and the old 3M polishing products such as #5711 Compound?

Pete Leoni Via e-mail

Having used both the old stuff and the new stuff, I prefer the new compounds and polishes.

The new products don’t have the grit the old products contain so they definitely do a better job of bringing out the shine. My concern was that the new stuff would force me to spend more time behind the buffer. However, that isn’t the case. Using the new 3M compounds and polishes as they were designed to be used has actually reduced the amount of time I spend polishing and shining these old rides. I like that.

Did You Need Body Filler For That Corvette?

I was wondering if the Corvette you used in the March issue required any body filler work. As I recall, those early fiberglass bodies were very imperfect.

Ray Sullivan Via e-mail

Yes. I didn’t show any of that work as I wanted to concentrate the article on the painting side of the project. I did a lot of filler work on all of the panels. The factory wasn’t too concerned about the smoothness of the body so I had to spend a lot of time perfecting the body before applying the dark blue paint.

(Editor’s note: Larry’s current article, on a new way to wire your ride, can be found on page 12.)