Natural stone signs are very good at weathering the elements. Our clients are advised before purchase of the material's suitability for use before purchase.
For example any types of materials that are highly polished calcium based (limestone, marble) would not be suitable for outside use.
This is because acid naturally accuring in rain water has a tendency dissolve the high gloss of the material.
In the majority of cases the only maintenance required is a gentle clean with a little washing up liquid to remove algee or dirt that can build up on the letters.
Warm soapy water with a neutral PH is advised.
Please do not use harsh oil based solvents as this can remove/bleed the paint into the stone.