XSLT 1.0 remove white space in elements before adding leading spaces

I have an element called <AccountName>A PERSON</AccountName>

which I am outputting formatted to a length of 18 using leading spaces like this:

<xsl:value-of select="substring(concat(msg:PayingBankAccount/msg:AccountName, '        '), 1, 18)"/>

Unfortunately, in the source xml, for some records that element is just white space like this:

<AccountName>                  </AccountName>

and in my output this seems to end up as just 9 spaces...I think I need to strip all whitespace in the document before outputting the formatted data with leading spaces and zeros

So given that there is no trim function in xslt 1.0, what is the best soltion to that?