Most film cameras have light seals at the door rims and mirror rest (SLRs only). These are made of some rubbery foams and they degrade over time. Considering that most of these tank-like cameras are over 20 years old, it is very likely that your camera is suffering from light seal degradation. Rotten seals will not perform as they should and leak light through the film door or the viewfinder.
Moreover, when degraded, the rubbery foam turns into a goo-like substance which will stain your focusing screen, mirror, or lens, contaminate the film compartment and even crumble over your film. It is very sticky and nasty, so it gets harder to clean when it gets worse.
But, you can find replacement light seals in bulk from ebay (search: light seal replacement, i.e., this one). Unless they are custom made, they are sold in bulk packs which you can cut/customize for your needs (most sets can fix over 3-4 cameras).
This instructable will show you how to apply them. I will start with my old Canon F-1 (I sold it long ago unfortunately). It was in great shape, except for the rotten foam light seals. They needed to be changed in time, so here is what we'll do next:
1. Identify and document the existing rotten seals,
2. Scrape off the seals, clean the surface,
3. Cut the new seal foam in correct size and shapes,
4. Apply the new seal foams in places.
There is not much risk in this operation, but you can still ruin the focusing screen, the mirror, and the shutter curtains if you are not careful. So be warned, and don't start if you don't feel comfortable.
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Step 1: Inspect the Existing Light Seals for Condition.
Look at the photos in this step, I have notes on them, and they are pretty self explanatory.
Step 2: Clean the Film Door Area.
Here we'll strip the goo-like rotten foam off the film door area. We'll use an alcohol based solvent to soften it, and strip it off using a bamboo skewer. In my case, the cleaning skewer came with the new foam set. Look at the photos below, I have notes on them.
Be careful to keep your fingers off the shutter curtains. They are very fragile, you don't want to ruin them.
Step 3: Clean the Mirror Box Area.
With a Canon F-1, this step is much easier to accomplish, because it has a detachable viewfinder prism. There is virtually no risk of contaminating the focusing screen (which is so fragile). If you are working on a camera with fixed prism, be very careful and do not ruin the focusing screen (it cannot be wiped, cleaned, and it is easily marred).
Be careful and do not touch on the mirror. It can be stained/damaged very easily.
Step 4: Measure, Cut and Stick Foam Seals for the Mirror Box.
Use a small ruler to measure the necessary dimensions for the mirror padding foam. Check out the photos below for notes.
Step 5: Replace the Foams for the Film Door Edges, and the Hinge Area.
Here we'll use the strip foam to replace the old foam for proper light sealing at film door area. Look at the photos below for notes.
Step 6: Finished!..
You have a light-tight camera now, go back to snapping awesome photographs on a wonderful medium: film!..
Any questions or comments, you are welcome to contribute.