Sealing and Stabilising Corroding Composite Stone Sills

[Jazzhands]

Hi, if this is in wrong forum please move to correct one.

My house has composite (I think that's what it's called e.g. they are cast rather than solid stone) stone lintels and sills on all the windows. The ones at the front get a battering from the weather and have corroded in places but moreover the surfaces are powdery and crumbling.
I am looking for a product that can be applied to the lintels/sills that will seal and stabilise the surface to help prevent further wear without discolouring the stone or making the surface glossy.

Can anyone recommence anything?

 

[Jazzhands]

I've reposted this in the Building channel but don't know how to delete this thread - can a mod delete if they see this please.