There is a need for a headstone to last a long time, in its lifetime its appearance should improve and at the same time it needs to weather and age gracefully.
Sometimes the weathering process is too rapid and the inscription on the headstone will become unreadable in a few years.
Factors such as location, size and depth of letter cutting and type of material all play their part.
If a porous Portland headstone sits in a damp churchyard under a yew tree, it will not have time to dry out and will quickly become covered in lichen and moss; standing on a windy hillside, the headstone will have more chance.
In this case the material will need to be chosen carefully.
Slate for example is non porous and would be a good choice.
It would attract a certain amount of organic matter in time, but this can easily be wiped off and the surface thus restored.
It is true that a 'polished' headstone will attract almost no organic matter and will hold its 'newness' for many years, but this is going too far; polished headstones are to be avoided firstly because they can look fairly cheap, but more importantly a gravestone needs to age to a certain degree and become part of its surroundings.
When headstoneshave to be a porous limestone for various reasons, the lettering must be large and boldly cut.
One cannot rely on paints that will not last.
The cut must be a V-cut and be adapted for different materials.