Do NOT put polyurethane on your logs, it will ruin them and turn them jet black. Slightly pressure wash or sandblast the logs first if possible, don't do so much where it makes the logs spinter. Then get shellguard-rtu (ready to use) and spray it on. This will prevent beatle bugs, termites, wood decay/rot. After this perserative is used, fill in any checks with ewood or mbalm. Then do a coat of any type of stain you need. The stain will also protect it from outside elements.