Learn Swedish: Vi kan se att utsidan är tvättad. - We can see that the outside is washed.

This is a perfect example of a stative passive. Using är (is) focuses on the resulting state: the washing is already finished, and we are looking at the clean result. If the sentence used the dynamic passive blir tvättad, it would mean the action is happening right now, as in 'the outside is being washed'.

Could we use the s-passive tvättas instead of är tvättad to mean the same thing?

No, the s-passive generally behaves like the dynamic bli passive. Saying utsidan tvättas means the outside is currently undergoing the action of being washed. Only the construction with vara (är tvättad) captures the completed state shown in this sentence.

Why is the participle spelled tvättad and not tvättat?

In a participial passive, the past participle functions like an adjective and must agree in gender and number with the noun it describes. Because utsidan (the outside) is a singular en-word, the participle takes the en-word ending to become tvättad. If we were talking about a t-word like fönstret (the window), it would be är tvättat.

Where would we put inte if we wanted to say 'that the outside is not washed'?