Friday, 09 May 2008 16:46

Hey Toby 5: Disposable Cups, STR Welding, and Shunts

Written by Toby Chess

Hey Toby—What is your take on those 3M disposable mix cups that fit on the spray guns?

              —Dave from San Diego

Dave—The 3M mixing Paint Preparation System (AKA PPS) is plastic cup with an internal plastic liner (internal filter) that allows the painter to mix paint in it. The PPS cup is attached to the spray gun and when the refinish process has been completed, the cup is disposed of, leaving the painter to only have to clean the gun body with the PPS adapter. Quoting from the 3M web site “3M™ PPS™ is therefore a cleaner, faster system, safe from outside contamination and offering considerable time and solvent savings on gun and parts cleaning.”

    I have to agree with the 3M quote, but I would like to offer another solution. You need to understand that this is a wonderful system, but it is little pricey and with things getting tougher in the body shop, I would like to suggest an alternative.

    I use 600ml (equivalent to about a quart) chemical beakers to mix my paint. The 600 ml heavy duty beaker costs about $7.00. Due to the nature of the glass, they are impervious to the paint. Clean up is a breeze. Put them into a gun washer and they are clean and ready to go again. I tell my painters to mix what they need and it shouldn’t be necessary to store the paint. Yeah, I have heard the painters say that they need some for touch up and I tell them to fill a touch up bottle at the time of the mix.

    Other cost saving hints: first, tell your painter to start with the light colors moving to the dark colors. Use the leftover paint as a ground coat. Secondly, set up a set of mixing cans for leftover basecoat and use them as ground coats.

    A couple of other notes: You can purchase beakers all the way up to 2 liters and if your people in the paint department are careful, you will not have to purchase any more mixing containers for a long time. Lastly, I think that cost of cleaner for the beakers (and paint cups) outweighs the cost of the PPS mixing cups and their proper disposal (you still have to clean your gun with both systems).


Hey Toby—I heard that Cindy Shillito of Imperial Radiator is back in the business. Got any contact information?

            —Al from Long Beach


Hi Al—You heard it correctly. I talked with Cindy the other day and she has teamed up with Complete Plus and they have put in a complete top of the line of radiators and condensers. She can be reached at 714-944-4028.


Hey Toby—I was reading an article on STRWelding and they mentioned to use a shunt. What is it?

        —Jeff from Los Angeles


Jeff—A shunt is a device which allows electrical current to pass around another point in the circuit. Remember, STR Welding is a process of passing electrical current through metal and the resistance from the metal creates large amount of concentrated heat at the weld site which, in turn, fuses the metal together (weld nugget).

    If you are using adhesive with STRW (aka Weldbonding) or weld thru primer, these products will stop the flow of electricity. To overcome this problem on the first weld, a different path for the electricity is needed. This is accomplish by the use of a very high tech tool—a vice grip. The current will flow from the electrode, along the metal (bottom) and thru the pliers, back into the metal (top) and complete the circuit at the upper electrode. After the first weld has been completed, the shunt is no longer needed because the current can now flow thru the spot weld. Remember to insulate the pliers that are used for fit up because we don’t want to create an additional circuit for the electrical current.


Hey Toby—What is copper weld thru primer?

        —Chuck from Chandler, AZ


Hi Chuck—Is it getting hot in the desert yet? Let’s look at weld thru primer. The original weld thru primer was zinc rich based material that was applied to underside of two mating surfaces that were to be welded together. I-CAR recommends that the weld thru primer be removed from the weld site, but not from the area surrounding the weld site. This procedure will improve the weld integrity and weldability (less spatter and porosity).

    When welding using a squeeze type resistance welder, the E-coat needs to be removed from the mating flanges and a coat of weld thru primer needs to be applied. The zinc rich primer has some electrical resistance to it. A final note: Chrysler does not recommend weld thru primer when welding.

    The copper in the weld thru primer provides superior conductive properties (for the electrical current) and it also minimizes the heat zone, thereby reducing distortion and welding splatter. It is gooood stuff!!!


Hey Toby—I have been reading a lot about these new steels and I keep seeing the term MPa’s. Can you explain it to me? Thanks.

             —Tim from Reno, NV


Good question Tim. MPa’s stand for Mega Pascals and it represents the yield strength of steel. Up to 200 MPa’s is the yield strength of mild steel, 200 to 700 MPa’s is the yield strength of high strength steels and above 700 MPa’s are the ultra high strength and advanced steels.


Toby is always looking for good questions. Send him your query to tcspeedster@yahoo.com 

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