If you go back to bare metal after derusting I'd suggest a couple of stages prior to applying stonechip or Schutz underseal. Get it as clean as possible and then apply two good coats Hydrate 80 rust inhibitor followed by two further coats of Zinc 182 primer.Seal any welds with good quality seam sealer , I apply a PU sealer initially and then a brush on top seal. Stock up with Aldi metal paint ( better than smoothrite ) next time it's on sale and apply a another two or three coats of the colour of your choice . Then two ( I prefer three ) coats of stonechip /underseal applied at circa 45 psi with a good Schutz gun allowing each coat to flash off before next. This will give you better rust protection than factory finish.
Re metal forming . If you are going to do it buy an inexpensive hammer and dolly set . Mine cost circa €30 and has done sterling service on my E24 restoration , well worth the small sum paid.
I use many different sections of scrap steel to form and shape steel but the most used is / are lengths of right angle . Used in conjunction with a stout vice you can create as many shapes as your imagination allows . Clamp them at different heights and you can create steps in the sheet , used with G clamps you can create flanged edges of whetever depth you require , space them apart using a suitable spacer you can create valleys or any shape of indentation you require . I also have a selection of steel rods of various diameters and shaped ground ends that are used to create more difficult or replicate original factory shapes . To create curves as said , using 20 swg sheet ( 0.9mm ) it is surprisingly easy to curve and bend . I have several pipes of diameters between 1 to 4 inch to create the shape required. To create the curve for the floor on the sketch I'd clamp the sheet between two right angle sections in a vice and then G clamp a pipe of suitable diameter onto one angle section and the gently pull the steel over the pipe to the required angle . I invested in a low cost sheet steel bender from Poland / E bay . Cost about £110 but has paid for itself allowing me to make my own sills and numerous flanged repair sections . I came close to purchasing a shrinker / stretcher but have found that small curves can be stretched out using gentle hammering , tighter curves are either made from welding two sections together or simply cutting V's out of the flange prior to curving around a suitable pipe .
You'll be surprised how quickly you learn to curve and shape sections and will soon be trying to make better and more accurate sections .
Just remember that every car is made up of various multi shaped panels that are joined by attaching two flanges together , as long as you can eventually mate these two flanges and weld them together , job done .
I couldn't buy a sill section for my e24 so made this in 18swg , I had to make it in two sections .
And likewise with floor sections as they are unavailable .
You'll definatly enjoy the welding and fabrication course and it'll give you so much more confidence in repairing bodywork.
Good luck and enjoy it
Edit , I should add that flap wheels on an angle grinder are far better suited at removing rust than wire brushing.
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Topic: Welding.... how on Earth do you shape chassis metal? (Read 4523 times)