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Are you sure it is rust? Degenerate (they turn to goo) foam light seals for the film door can gum up the latch and make the door hard to open. Bad seals are the most common issue for 35mm film SLRs. Fortunately they are easy to replace. You can pay to have it done as part of a general CLA (
Clean,
Lube,
Adjust) or you can do it yourself using a kit of pre-cut foams.
For the best CLA at a very fair price, I recommend Eric Hendrickson in Tennessee. His Web site is at:
Eric Hendrickson: Pentax Service
For seal/mirror bumper foam kits, the best source I know of is Jon Goodman. The kits are $10 including shipping. Send me a private message on this site for Jon's e-mail.
Steve