Patchy spots on cedar cladding on garden room after varnishing

[robbefowler]

Hi there,

I recently had a cedar cladded garden room sanded and varnished. The cabin was installed about 3 years ago and at that time was finished with 2 coats of OSMO clear UV oil. I wasn't around for the recent repainting of the room but I did see the decorators sanding back the wood to prep (not sure what grit they used). WhenI arrived back from work after their first coat they mentioned that they were't using OSMO oil but Johnstones clear varnish which would give a similar result. I didn't think too much of it but the finish achieved (see pic below) is awful. There was some discoloration prior to them starting work but only on certain patches of the cedar where water had been dripping (looked a bit whiter than the rest of the board but wasn't horrific) but tbh I can't take my eyes off the large patches……anyone got a suggestion to remedy?

It isnt on all the cedar boards and I kind of know where this will go (involves me resanding and refinish with OSMO) but I wonder if anyone has seen this before on your own projects? The painter did suggest a stain which I didn't want as it would be there forever if they applied it. If I do resand and refinish does anyone have a suggestion on what grit sandpaper to use? I would most probably use the OSMO oil previously applied.

 

[Wayners]

I've not seen anything like that before.
I'd say some contamination on surface looking at the shape and two area's close to each other.

Re grade.
120g
Then 240g

I think it would be worth contacting the varnish technical helpline for both companies for advice before proceeding, and reading the application data sheet for both materials.