What causes it to turn milky white after a few months?
It could be an efflorescence problem: The moisture may be pushing up salts, which may not have been previously visible due to the degradation of an old sealer. The gloss can wear off of old sealers without the actual sealer wearing off. The old seal should have been stripped. To remove the old seal: Test strip a small area, clean with an acid wash, neutralize, and allow to dry. Then, reapply the sealer. In colder climates, such as CO, you may want to do the test first, and then let it go through the winter. It may look funny, but you cannot apply any sealer when it gets cold, and this will tell you if this would be the fix.
