There is quite a lot of nonsense written about the cleaning of lens filters making it appear far more difficult than it need be and recommending the use of different types of expensive cleaning fluids which in their turn give rise to dire warnings about the possible damage these can do.

Lets face it, all we want to do is to make sure that the filter is undamaged and be able to clean it simply and safely, not to mention cheaply.
The following suggestions should allow the safe and accurate checking and cleaning of most filters.

CLEANING
Unless a filter is absolutely filthy, the most one needs to do is to breath heavily on the glass and then wipe away surface smears with a plain cotton pad. A cheap and easy way to make a good filter cleaner is to take a 5inch square of soft cotton material (gentlemen, a piece out of an old pair of M&S Y fronts is ideal) Lay it flat and place a good wad of cotton wool in the centre and form the whole into a round, firm soft pad, similar to that used for french polishing, tying the top with strong string or wire.

 

The cotton pad may leave a few stray bits of fluff on the glass, so finish off with a microfibre cloth. These are easily obtained for around £3

If the filter is dreadfully dirty, then just wash carefully in a weak solution of plain soap and water, dry and then carry out the final clean as described above.

A WORD OF WARNING: Some specialist effect filters rely upon two pieces of glass to obtain the effect. DO NOT IMMERSE THESE IN ANY LIQUID because it will run between the glasses and be very difficult to remove causing staining and damage to the filter glasses.

CHECKING THE FILTER
One of the two most important things to check when purchasing used filters is the glass. Holding a filter up to the light may reveal some marks but not all.
 Position a desk lamp so that it shines directly onto a matt black surface. Holding the filter about 6 to 8 inches above the surface, making sure that the light does not reflect in the filter glass, gently move the filter about so that the whole area is inspected . The smallest and previously unseen scratches and blemishes will be revealed.

The second most important matter is the condition of the ring. Most filter rings are made of aluminium and are subject to damage by dropping or by being forced off the lens when they become stuck.
The most important part of the filter holding ring is the screw thread that fastens it to the lens barrel. If this is bent or damaged the filter is useless. Check the screw thread on opposite side of the filter ring, this is the side where you would screw in another filter on top if so desired. This is where most dents and  damage are found because this is the exposed side of the filter that takes the knocks and scuffs. If this is damaged and the rest of the filter is fine, then provided you will not require to screw another filter into it, don't worry about it.

Continual use sometimes wears the lettering off the perimeter of the filter but unless you are particularly fussy about appearance then ,again, don't worry about it.

If your filter becomes stuck on the lens, usually due to over tightening, don't grip the edge and try and force it undone because the pressure of your fingers will only tend to squash the aluminium frame making the situation worse. Place the palm of your hand flat on the filter and with slight pressure turn your whole hand . This spreads the load across the whole of the filter frame and in most cases this will resolve the problem. If a filter is really stuck one can obtain a pair of filter spanners that grip the whole of the outside of the filter and the lens barrel allowing more force to be used. Prevention being better than cure, DON'T OVER TIGHTEN THE FILTER IN THE FIRST PLACE!