Wash. Rev. Code § 84.64.180
Deeds executed by the county treasurer, as aforesaid, shall be prima facie evidence in all controversies and suits in relation to the right of the purchaser, his or her heirs and assigns, to the real property thereby conveyed of the following facts: First, that the real property conveyed was subject to taxation at the time the same was assessed, and had been listed and assessed in the time and manner required by law; second, that the taxes were not paid at any time before the issuance of deed; third, that the real property conveyed had not been redeemed from the sale at the date of the deed; fourth, that the real property was sold for taxes, interest, and costs, as stated in the deed; fifth, that the grantee in the deed was the purchaser, or assignee of the purchaser; sixth, that the sale was conducted in the manner required by law. And any judgment for the deed to real property sold for delinquent taxes rendered after January 9, 1926, except as otherwise provided in this section, shall estop all parties from raising any objections thereto, or to a tax title based thereon, which existed at or before the rendition of such judgment, and could have been presented as a defense to the application for such judgment in the court wherein the same was rendered, and as to all such questions the judgment itself shall be conclusive evidence of its regularity and validity in all collateral proceedings, except in cases where the tax has been paid, or the real property was not liable to the tax.
RCW 84.64.180