the Silver Arrows Mercedes was white, but it was 751 KG, and the entry regs said max weight was 750 KG, so they removed the paint, leaving the silver metal underneath and it weighed 750 KG
you should remove the three circles; those decals are just added weight! Plus, the wind resistance incurred with the half milimetre depth of the decal!
:P
EDIT:
To enforce the minimum ride height. The cars lower quite a bit at high speed, so you made have a car that is at the minimum height, but once youre in motion it lowers because of the downforce being applied to it. The plank is a way to keep track of it; when the car bottoms out it scrapes away at the plank. After a track session the planks are all measured for thickness, if your plank is too thin than your car was sitting too low when at speed and you are penalised.
I think the rule is something like you have to have Xmm of the wood left at the end of the race or you run too low. Came in about the time they tried to limit ground effect from what I remember.
But I don't know about the sealant--if any--or species of wood (this would make a big difference in weight and water resistence (you would want to have the densest wood possible and take away what ballast you must, as the plank is lower than the ballast, and it would wear better (then that's only if you had a choice)))
Also, about that Jabroc plank on the bottom of the F1 cars, I think they should be fastened to the car with giant titanium bolts, so they will scrape all over the place, and make some pretty sparks.
F1 teams were/possibly still believed to use Depleated Uranium for bolts in the bottom of the cars. The material that is used for the rounds of the main cannon of the tank-hunting A10 warplane are about the densest material thinkable, thus lowering the cars' center of gravity. When almost half of the ~530(?)kg/1200lb weight is Ballast, the engineers come up with some incredible tech.
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I try not to think too hard about mass dampers or ferrari's clutchless transmission, they make my head hurt (mainly the former).