Comment by csours | Hacker News Reader

Comment by csours | Hacker News Reader

I used to work in an automotive paint shop. Another point of interest:

There's a limit on the number of tries you have to "repair" the paint job, because the vehicle has to go through the oven again to bake the coatings. The trips through the oven affect more than just the paint. Some panels are filled with foam; there's internal sealant and glue, and external sealant. All of those things don't really love being baked too long. For the shop I worked in, we got 2 tries to repair.

The main limit on repairs though? The windshield. Windshields are glued in with specialized urethane 'caulking'. They have to stick to the paint, and the paint has to stick to the body. That is only certified reliable with less than 5 trips through the ovens (in my shop).

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Paint is the hardest thing to get right in the whole assembly process, because there are so many process variables. Temperature and humidity (dew point) are the biggest besides the actual paint mix, but there's also unexpected problems from every direction. Our solvent paint process (pre-2013) was very vulnerable to siloxane (silicone) lubricants. Someone in IT applied some lubricant to a label printer and ruined hundreds of paint jobs. Livestrong bracelets also ruined at least a hundred paint jobs. Someone did weekend work on the roof? Congratulations on your dust quality spill (spill means a bunch of problems).

All of the parts of the paint have to work together, and that includes things that you don't think are paint. The body is just dumb metal, right? Of course not. The metal panels have oil on them in stamping, and they MUST stay oiled right up to the point where the body takes it's first bath. If too much of the oil has evaporated, then you start getting corrosion, and in many cases you won't be able to tell until the vehicle gets to the customer.

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Paint shops have the most complicating routing of any part of automotive assembly. The electronic quality and build data has to be synchronized with the physical process, and that data package can be updated by several different systems. Then either IT or OT (operational technology) can make a routing decision, or an individual can use a key switch to manually change routing. "Why did this vehicle go here" is not a straightforward question to answer sometimes.

> The metal panels have oil on them in stamping, and they MUST stay oiled right up to the point where the body takes it's first bath.

I toured an assembly line a few years ago. The guide told us that in the 1970s they had teams of people watching iron/steel prices, when they thought the prices where at the low they made a massive order of all iron that factory would need for the next few years. When the factory when to just-in-time (sometime in the 1980s or 1990s) they couldn't do that anymore - but they also got to get rid of all the equipment to de-rust all that stored iron: since every raw material was shipped out as a final assembly in a week there was no time for significant rust to form and so they avoided a lot of problems.

JIT assembly has SO many non-obvious implications. I had not heard of this one, thanks.

Any reason why an automobile manufacturer couldn't buy a steel futures contract to fix the price?

That's effectively what they were doing with the bulk order, right? By just trading the overhead of the future with the overhead of derusting.

I don't work in that area, but I suspect that is done. Auto makers also have the ability to order months in advance so they can yet discounts because the steel makers know what orders they need to fill each week instead of a lot of overtime to fill a large order.

Thisbis done, of course. Risk with securing long-term prices so is that those prices might be higher than short term ones for the same future period. So usually, admittedly it is a while I did raw material procurement and aerospace is different, you use a combination of both.

You can still get squeezed so either way. But that has close to nothing to do with JIT, JIT just concerns the delivery bit (over simplified of course).

Futures contracts are not free

Nobody said they were?

This is fascinating, thanks. Can you elaborate more on how the label printer and Livestrong bracelets ruined paint jobs? Was it something they emitted through the air? How would that be in concentrations to affect the paint job, what would happen to the paint?

Silicone oils caused the paint to "run away", like putting soap in a greasy pot.

Droplets were enough to cause problems - there's no hiding problems on a car's paint job and customers are VERY demanding.

So it was workers wearing the bracelet that caused the problem, not end customers?

Yes

Silicone has very low surface tension, imagine water on a non stick pan. If paint encounters a droplet of silicone it will shrink back on itself and form a hole around the contamination.

That sounds fascinating and complicated. I scratched my car a few years ago and decided I wanted to DIY a chameleon paint job instead of taking it to the shop (the car is worth less than the paint job if I pay retail). I’ve been slowly acquiring all the gear I need like a forced air respirator and sprayers.

I was wondering how realistic that really is to DIY or if I’m kidding myself. Sound like even if I rented a stall in a paint shop for a few weeks, I won’t have the time to finesse the technique to get a good end product, especially with how much more finicky I understand the chameleon paint to be.

My understanding about chameleon paint is that it's very hard to match if it's not all painted at once. I think the difficulty may depend on your expectations for the final product.

Yep from what I’ve read I’ll have to do the full body in a single shot after spending several hundred man hours sanding down the old paint job and prepping the surface. I’m hoping in Southern California’s weather I won’t have much problem with rust between the prep and actual painting.

I don’t care so much about getting the matching perfect - “it was my first time” is a perfectly cromulent excuse in my social circles - but I do want the paint job to be good enough so that I’m not redoing it every year or two (I hate sanding). What should I exepct?

I don't think I can help much more, sorry. The shop I worked in was completely automated and worked on new builds. In most ways, it's much harder to do a one-off repaint of an assembled vehicle.

Only advise I can give is to get some cheap paint and some kind of panels and start practicing.

Sometimes the local tech school will give you a deal on painting. Students do the work and it will have flaws, but better than you can do without practice

Maybe vinyl wrap would be a more approachable solution? https://www.rvinyl.com/Chameleon-Shade-Shifter-Vinyl-Wraps

Thank you for this comment. I enjoy hearing about intricacies of engineering processes-- particularly those that sound "easy" on the surface.

This gives me a deeper appreciation for cars with plastic body panels. I also wonder if DeLorean Motor Company took any of this into account when they designed the DMC-12 with stainless steel body panels.

Plastic panels are a whole bucket of issues in themselves. As far as I know, the color matching plastic panels are painted using a different process. That means the the front and rear fascia (bumper) of your vehicle are painted in a different plant (usually) from the rest of the vehicle. The colors have to match to a very high degree.

And yes, that's a big reason why they went with stainless. Stainless steel does corrode, just much less visibly than regular steel. It also has different physical properties like work hardening etc. It's also VERY expensive. Salty napkin math says $10,000 more for the Cybertruck.

> Plastic panels are a whole bucket of issues in themselves. As far as I know, the color matching plastic panels are painted using a different process.

I was thinking about "molded-in color" panels like the old Pontiac Fiero. That makes sense, though, re: painted plastic panels. They'd be a whole different ballgame and there's, I'm sure, interaction between the paint chemistry and the plastic chemistry that's probably pretty hairy.

Color matching seems hard, I almost think it is worth making the distinction clear and not even trying if you have different materials. I've watched my girlfriend hunt down matching dye lots of yarns to finish gifts when she runs out too early.

I bought 2 pairs of the same shorts a couple years ago, one in blue and one in olive. Both were used almost interchangeably in terms of exposure to sun, dirt, washing, and salt water. But on the green ones a section of the waistband is clearly differently colored where on the blue the colors still match.

The DMC-12 had stainless steel body panels, but it had a plain ol' carbon steel frame (designed by Lotus and similar in design to the Esprit) with an epoxy coating, and they have NOT fared well over the years. You can buy a new-build stainless frame for it, it only costs as much as a new regular car.

https://deloreanindustries.com/8-1-0-stainless-frame/

Same with old Land Rovers, people belive they don't rust being all aluminium. Chassis are steel, and those rot. Plus some other bits and pieces. New galvanised chassis are available, getting the body mounted is the fun bit so.

The epoxy coating seemed like a really bad idea when I learned about it. That’s a recipe for trapping moisture and causing corrosion.

You were painting bodies with windshields already bonded onto them? Seems like a good idea to adhere windshields after paint.

I got the feeling they were in an auto body shop, where the paint might be trashed and the windows might be fine.

Nope, the windshields go on top of the paint, but the requirements for the windshield affect the paint process.

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