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Do you have a lens collimator and other equipment necessary to ensure proper alignment when reassembling the lens? If not, take it to a good camera shop and have a properly trained and equipped professional do the job.
The dreaded fungus is something that bothers every photographer. I have suffered from it before, and every coming rain gives me constant worries of possibilities of fungus growing somewhere inside my lenses. I am more vulnerable to fungus than most people because I don't think much about taking out the camera to get some good shots even if it has been raining a bit, and have often got plenty of rain drops on the camera and the lens therefore. Where does fungus comefrom?
Fungus spores are everywhere and germinate under suitable environmental conditions:
Relative humidity of at least 70% (more than 3 days)
No or little airflow
Nutrients (textile lint, traces of grease, varnish, dust and dirt)
Temperatures between 10 and 35°C
How can fungus be avoided?
Silica Gel. This seems to be the easiest and common way to avoid fungus, but I haven't had much success with it either.
. Store the lenses in an airtight container. Again, you may need to put a few Silica Gels in the container. Putting too many of Silica Gel is known to dry out the lubricants in the lens, so you need to use your discretion with quantity of the Silica Gel.
. A popular idea is to install a low wattage tungsten bulb in the closet where you keep the lens. This will keep the place warm and dry, preventing the fungus. This is known to work.
. Store the lens in transparent container. Fungus is known to fear light and love darkness. Also make sure that the lenses are not stored in leather bags, as leather can easily attract fungus.
And finally, of course, the best way to prevent fungus is to get out and shoot more often.
After the work is done, immediately clean the instruments. If possible, you can use a fan or blower to facilitate evaporation of surface moisture.
Well, that's a great lens and most definitely worth the cleaning/service. I don't know where in the world you are but fungus growing inside a lens is only heard of in high humidity environments and can happen fairly quickly. Are you sure this is a fungus on the inside and not just a clouded smear on the front element? If the lens is having a problem then the camera won't be to far behind. To answer your question more directly "what should I do" my suggestion is to send the lens to a authorized Tamron service/repair center/depot for cleaning. After which get a few of those little moisture absorbing packets and place them in with your photo gear to help reduce the moisture.
It is lens fungus and you're correct in your assumption that it's uneconomical to professionally repair on your lenses. But it need to be pretty bad before it affects image quality so don't junk them just yet.
Some types of lens fungus can be killed simply by leaving the lens out in sunlight, but I'd suggest that it's better to leave them exposed to the indirect sunlight on a bright overcast day to prevent any chance of sharply focussed light burning the inside of the lens. Killing the fungus won't remove the dead stuff but it prevents the problem getting worse.
Fungus etches the glass. This cannot be fixed by just removing the fungus. The lens has to be resurfaced, calibrated, coated and well, you get the idea. lots of expensive precision equipment and a jig to recalibrate the focusing after you do all that. Another 70-300 on Ebay is much less expensive.
That is a manufacturer repair. While you may be able to remove the fungus, the fungus may have etched the coating on the lens and still show . The best way to fix that is replace those lens elements that are effected.
Try Adorama camera in New York. They are good to deal with. Binoculars.com also used to carry parts but I have found since they sold to a conglomerate their customer service is wanting and i switched to Adorama. They also post international.