I own a small apartment block, and I clean mildew off of silicone caulk
all the time.
If the mildew is on the grout, you're best bet is to use phosphoric acid
to dissolve the surface of the grout until the grout is clean, and then
use a grout sealer to seal the surface of the grout so that it doesn't
If the mildew isn't too bad on the silicone itself, you can clean it up
by mixing Borax (which you can find in the laundry detergent aisle of
any supermarket) with bleach to make a thick slurry. Use a large spoon
to scoop up that slurry and hold it near the silicone and then a small
spoon to spread the slurry over the silicone. Then, cover that with
Saran Wrap or any cling wrap for a few days. When you remove the cling
wrap, the silicone caulk should be white as Manitoba snow.
Now, in the above instructions, you'll get the same cleaning effect
regardless of whether you use Borax or any powder since the active
ingredient is the bleach. However, my experience is that using Borax
makes for a stickier slurry that will stick to vertical surfaces much
Also, if the silicone is really badly mildewed, the above procedure will
only lighten the mildew, and you'll still have brown stains on the
silicone. In that case, the only option is to replace the silicone.
It's common to have people who have replaced the silicone around their
tub find that the new silicone caulk won't stick. The reason why
they're having a problem is because they haven't removed the old
silicone caulk COMPLETELY. So, what they're trying to do is get the new
silicone to stick to a very thin film of old silicone caulk that is
still on the tub and tile. THE BEST WAY to remove the old silicone is
1. Cut off as much as possible with a single edge razor blade gripped in
the jaws of a pair of needle nose style locking pliers.
2. Apply "Silicone-Be-Gone" silicone caulk remover which you should be
able to buy in the caulking aisle of any hardware store or home center.
Silicone-Be-Gone is nothing more than gelled mineral spirits. It does
not DISSOLVE the silicone caulk, but causes it to swell and become quite
soft so that it can be more effectively removed by mechanical means.
3. Scrape off as much of the swollen silicone with the razor scraper
described in Step #1.
4. Repeat Step #2 and then scrub the area with a green Scotchbrite pad
of the kind sold in grocery stores as pot scrubbers. (If you have a
plastic tub or shower pan, use the white Scotchbrite pads for delicate
scrubbing. The white Scotchbrite pads don't have any abrasives
impregnated into the nylon fibers from which the Scotchbrite pad is
Silicone-Be-Gone will emuslify in water, causing the water to turn
whitish. If you've removed all of the old silicone from the tub and
tile, the new silicone caulk will stick to both the tub and tile like
chewing gum to the underside of a church pew.
And that's it. Clean as much of the mildew off the grout as you can
with phosphoric acid, which you can buy as a toilet bowl cleaner from
any home center of place that sells janitorial supplies. Then clean the
silicone as best you can with a borax/bleach slurry or remove it
entirely using Silicone-Be-Gone and replace it.