Tax Year Prior to 2020: If I have a home equity line of credit secured by my main home and I use all the proceeds to build my second home, is the interest fully deductible?

According to pub 436, if proceeds from a home equity loan are used to build or improve your main home or second home, the interest is fully deductible if the loan is secured by the home being built or improved.  However, if the loan is secured by the main home but used to build the second home, is the interest now subject to the limitations described in 936?