How do you deal with warped parts on a big 1/35 kit?

The best approach here is likely controlled heat and gentle persuasion rather than just dunking them in boiling water. Since these rails are long and under residual tension from the sprue, rapid temprature change can cause the plastic to shrink unevenly or warp further. Hot water soaking is good for initial softening, but you need to follow up with a way to hold that corrected shape while the plastic cools and sets. Try soaking the bowed sections in very hot water, just enough for the plastic to become pliable, for about thirty minutes. After soaking, you need to carefully remove them and then clamp them or secure them flat between two perfectly level surfaces. If you don't have specialized clamps, even a large, heavy piece of wood clamped onto your cutting mat and weighted down with non-staining objects will work while you allow them to cool completly and stabilize. If the bow is too severe to correct with heat, the gap filling is your backup. I've found that simple putty works fine, but applying it in thin layers is crucial. Just jam a small amount of filler into the gap, let it cure enough to grab, scrape off the excess with a sharp hobby blade, and then repeat the process. This prevents huge lumps. Once the gap is filled, you still need to sand it down to a smooth finish before primer, or it will definatly show up under paint. Most people find that the slight gap is negligible if the surface prep is clean and the underlying plastic is solid. If the rail is structurally sound after the heat treatment, the putty should blend well enough to disappear under a few coats of flat grey.

Had the same issue with a long bed piece on an older kit that sat warped in my stash. Hair dryer on low for a minute or two worked faster than soaking for me, then I pressed it flat between two chunks of glass from a picture frame with weights on top. Held shape fine once cool. Also, for the joint itself, I drilled through both rail sections and pinned them with a bit of brass rod before gluing. Kept everything lined up while the glue set and added some strength to that long seam. Might save you from chasing a twist down the whole assembly.

Gotta disagree with the part about putty disappearing under a few coats of flat grey. Flat paint is the worst finish for hiding filler, it scatters light and shows every little ripple and sanding scratch, expecially on a long straight edge like a launcher rail where your eye follows the line the whole way down. Satin or gloss hides seams far better. If it were me I'd skip the filler debate entirely and just scratch-build the worst rail from strip stock, it's a simple straight profile with no tricky detail to match, and you end up with a true edge instead of a repaired one.

That Rubezh kit has been on my shelf for years, still bagged. My trick with long rails goes back to the old days: lay a bead of liquid cement along the inside face and let it flash off half set, then hold the joint togather while it welds. The softened plastic lets you nudge out a millimeter or so of gap without any heat at all. Worked on my Spandrel rails and on a Scud launcher back when Trumpeter first showed up over here. Also, scribe a centerline down the back of each rail with a sharp blade before you join them. Cheap insurance against ending up with a twist you can't un-see once it's on the turntable.

Reading this becasue I had a similar bendy part problem on a tank kit last year and never fixed it, just glued it crooked and felt bad about it every time I looked at the shelf. One thing I tried that actually sort of worked was pressing the piece under a stack of freezer packs from the kitchen. Cold for a whole day, then weights again. No idea if that is a real technique or just luck. Does anyone know if freezing plastic does anything at all, or am I making that up?